[ryan]

I think you are correct. I was typing up a message to the effect of I thought that you only needed CJX & HM/LT, but the more I think about it, the more I think that it DOESN'T work with the CJX ab piece.

So you can have the primary three forms, or you can have your choice of 2 of any of the other forms. You'd either need another HM/LT set or the CC/JJ set to get a 3 ab piece with the pegs. I THINK it's just the one piece that CJX doesn't have that is needed. I think the CJX hips work fine, because of the change needed for the Extreme form. But I might be mistaken.

Yes, you would have a ton of extra parts, enough to make 1 additional figure (but he'd be backwards JokerCyclone) and nearly enough to make a 2nd additional figure.
Really, in your scenario, the only thing the CJ/X set gives you is the CJX form. CC/JJ has everything needed to do the form changes with the HM/LT set, so you could get away with just buying the 2 sets, unless you really want CJX, which I think it's a great form.

I have always intended to get CC/JJ so I could leave CJX alone and have 3 figures for full form change abilities. But I've always found other stuff to spend money on before that. I just display mine in the primary 3 forms.

I definitely recommend them, they're fun figures, the form changing is fiddly, because you break the figures down to their component piece, it's not a rip it in half and stick it together again like the WFC. But imo, the Fang Joker / Skull set is the most awesomest, but FJ is also Double's best form.