[Matrixbeast]

Random thought crossed my mind. I got curious curious, how often do the main mecha and sixth ranger's mecha have a combination? While it's typically seen as a given, does it happen as often as we assume it does each season?
And I also wanted to know how far one could stretch the idea of a combination. Does standing on a mech mean they've now combined?
Zyuranger/MMPR S1 has Zyutei Daizyujin/Mega Dragonzord (And the Ultra combination), so that's a Yes.
Dairanger/MMPR S2 has one that's debatable. Won Tiger can't combine with RyuuseiOh, but it does have the Heavy Armor Qi Palace...Which involves Won Tiger sitting inside Daimugen, and the individual components of DaiRen Oh standing on top of Daimugen in their Qi Palace formation. Does this count as a combination if no one is combining? I dunno, so I'll put a tentative Maybe? for this one.
Kakuranger/MMPR S3 is the first one that truly matters on whether or not you're going with PR or Sentai. For PR, it's an easy Yes, since all 5 Ninja Zords plus Tommy's Falconzord. But Kakuranger's sixth stand in is the extra hero, Ninjaman, who can't combine, making it a No.
Ohranger/Zeo holds another contentious option. King Pyramider and Ohranger Robo's combo is just the components of Ohranger Robo standing on King Pyramider like an aircraft carrier, while Super Ohranger Robo's combo is just it standing inside Pyramider. No combining, just carrying with extra steps...So...Maybe?
Carranger/Turbo is a clear cut No. Signalman's robot can't combine with rangers'. And even considering Turbo's Phantom Ranger as a full blown ranger, giving the Rescue Megazord Artillatron's cannons a combination does not make.
Megaranger/In Space pushes things a bit. While the Mega Winger can't interact with the Astro Mega, it can with the Mega Voyager. However, it's only able to equip it's wings and nothing more. Hard to call it a real combination, so much like the Rescue Megazord getting some cannons, Mega Voyager getting some wings I wouldn't call a full combination. No.
Gingaman/Lost Galaxy is lacking a sixth ranger, and the stand in cannot combine with any of the mechs. Easy No.
GoGoV/Lightspeed Rescue once again varies depending on which version we're going with. GoGoV lacks a sixth ranger entirely, not even having a stand in extra hero, so No. However, Lightspeed has the Titanium Ranger, whose official zord is the Max Solarzord, which can combine to make the Lightspeed Solarzord, so...Yeah. Even if footage of Ryan piloting the thing is rare, it's still his.
Timeranger/Time Force is a No. While Time Robo can ride V/Q-Rex, it's not a combination, just one mech hitching a ride on another.
Gaoranger/Wild Force is a surprising No. Despite the play feature revolving around part swapping, there's no real way to combine all 6 rangers' mechs.
Hurricanger/Ninja Storm is close. However, it falls just short. Tenkuujin and the Dolphin zord don't seem to get along, as no combination makes use of both, leaving it out of the final 2 combinations. No.
Abaranger/Dino Thunder is also an unfortunate No. Brachio participates in no combinations, merely hauling the others into battle (And being used as a ramp once or twice).
Dekaranger/SPD finally nets us a 6 person combination, not excluding any rangers for Super Dekaranger Robo. Yes
Magiranger/Mystic Force Once again opts to not, as Travelion sticks to itself. No. Fun fact: For the toys, Travelion's torso can replace Magiking's, but that's still axing Phoenix and Fairy (And the other cars), so it's still no.
Boukenger/Operation Overdrive. Despite having the impressive 10 part Ultimate Daibouken, it doesn't utilize any of Siren Builder's mech to do so. So No.
Gekiranger/Jungle Fury is also an easy No. Regardless of Touja or Fire, the Rhino and Red's mech are put together, while the other 3 rangers' mechs just run alongside it (Also, not a six man team anyway, but even including Rio and Mele, the other 3 mechs just kind of running still invalidate it)
Go-Onger/RPM A very strong Yes. G12 is one of the poster boys for clusterfuck combos, and for good reason. And all 7 rangers were already together for G9, so an even bigger combo is just icing on the cake.
Shinkenger/Samurai is also another Yes, though much weaker. All 6 rangers' mechs come together for Daikai Shinken Oh, though by the time we reach Samurai HaOh, pink and yellow's mechs are just kind of hanging off the rear wheels? Eh, still, they achieved a full blown combo earlier, so it's fine.
Goseiger/Megaforce is another Yes. Back to back, fantastic! Ground Gosei Great makes use of all six rangers, as well as an additional Headder for good use. While their final mech, Gosei Wonder, can't interact with Ground, they still managed a full combo of all 6.
Gokaiger/Super Megaforce is most definitely a No. While their Kanzen Gokai Oh is impressive, it eschews the other 4 Gokai machines and 2/3 of Gokai Silver's GoZyuJin.
Go-Busters/Beast Morphers is a very solid Yes. Great Go-Buster goes through extensive methods to use everything, even to it's own detriment of looking chunkier than a fat kid at a golden corral.
Kyoryuger/Dino Charge is a curious No. Despite it's abundance of various combinations, there is no combination that uses all 5 main rangers and Kyoryu Gold. Hell, it takes until their final combo to even use all 5 main rangers in one combination.
ToQger needs the help of the secondary mecha, Diesel Oh, when combining with Build Dai-Oh but they get there. Yes.
Ninninger/Ninja Steel is an easy Yes. I'm not so pedantic as to consider whether the individual shuriken should be included.
Zyuohger is another easy Yes. The first 9 cube animals all come together. Their final form pushes it by excluding the trailer portion of Cube Rhino, but Tousai King gets it done.
Kyuranger/Cosmic Fury ends our streak. The closest we get is Kyutamajin, but that excludes nearly every mecha, only using the Kyutama. So No.
Lupinranger vs Patranger is a very curious situation. Now, we obviously have Good Cool Kaiser VSX, a combination that uses all 7 rangers mechs. However, viewing each team as individuals, there is no combo between Lupin/Pat X's trains and the normal mecha. However, there is still a combination that makes use of all the main mechs, even if they have to be with their enemies to do so, so Yes
Ryusoulger/Dino Fury continues the tend of having a highly customizable system, and not utilizing it to it's fullest. Once Mosa Rex starts appearing in combinations, the other rangers' mechs tend to be excluded, unfortunately. No
Kiramager is another No, with Gigant Driller doing it's own thing.
Zenkaiger continues on with the lack of full combos. While we get a 5 man combination at the end, Crocodai Oh is never modified to interact. Sad. No.
DonBrothers though, has a hell of a final combination, making use of both of Jiro's mech and the Phoenix. Yes
King Ohger then takes it even further, not just making use of all 6 rangers, but even each auxiliary mech too, for a huge combo in God King Ohger. Yes.
Boomboonger has us asking if one mecha squatting as another stands behind it to fire a big attack counts as a combination. I don't consider it, but others may, so...Maybe?

Going through, it's curious to see that, if only going by Sentai, we don't have a definitive 6 ranger combination for 9 years (Ohranger to Dekaranger. Even further if you're like me and you're hesitant to count Ohranger's combinations).
We also max out at 2 different 3 year streaks of full combos.
Additionally, despite introducing the sixth ranger over 30 years ago, only 11 seasons actually feature a full 6 ranger team combination.

I find it interesting that it's not as common as we assume it is.