Killer first appeared in episode 18, Abare Pink's episode was 38.
So no.

Oh... *googles* my mistake, sorry. :o

Gekiranger's comparison doesn't really count. Gekiranger is much like Liveman and Go-Onger, with a core team of 5 that just happened to have 2 of it's members appear latter.
For Abaranger, Killer is most definitely the 6th. It's really rather hard to say, but for 6th rangers like AbareKiller, it's just a case of "You know it when you see it." As poor as it is to describe it as, that's really the only way proper, since I know no one felt AbareBlack (Who morphed at, what, episode 5?) was Abaranger's "6th" ranger.

He fills out the role of the sixth despite there being 5 Abarangers.

It's about narrative role really. AbareKiller isn't a 6th by number but fits the role that a sixth ranger plays and has the usual characteristics (turns up a good while later but sticks around persistently, slightly different aesthetic, etc). Geki is a strange case though.

Geki's "Sixths" are Rio and Mele, yet they're in the Extra Hero category.

I wouldn't have said Rio and Mele fit the usual criteria for sixths personally. Extra makes more sense for them.

Extra heroes aren't really 6ths, though. They're just as described on the box. They're just extra heroes to the team. Usually the classification is that they're not really rangers to begin with. The only ones in a grey area are GoseiKnight (A header who is classified as a 6th, despite not transforming into a ranger) and Rio (Who I just assume they didn't want to separate from Mele, how cute~).

Well, if you look at Dekaranger, Dekamaster appeared before Dekabreak and Break is the '6th' ranger, yet Dekamaster is considered an Auxillary, maybe thats the way it should be, extra ranger and auxillary, extra joins the team occassionally but sometimes drops in and out from the main team but an Auxillary generally is on the sidelines unless they're really needed

Well, yeah. They do that.
They're officially classified as Extra Heroes (Signalman, Rio, Mele, Black Knight, Zubann, Gunmazin, etc etc) and Additional Rangers (The other 4 Dekarangers, the other 4 Kyoryugers Princess ShinkenRed, etc etc)