Thank you. Also: Wow...I mean, I knew that many figure lines had a lot of variation (my very small Star Wars display has a mix of 5-POA, full body articulation, and one Dooku that just waves his lightsaber when you press his legs) but I had no clue it was that complicated.
Oh, well, worse case, I can always just pose them around him and I'm pretty sure that rear turret could be manned by anything. I brought this up as I noticed my BAT had a full range of articulation, but my 'jogging pants' Zartan (I don't know the proper name or generation) was an O-ring style, and then I started seeing the peg hole differences and it snowballed from there.
I want to get the H.I.S.S. Megatron as while I recognize the weaknesses, it's a fascinating design and looks big enough to take on Ghostbusters Optimus, and I'll evaluate others as I see them, but trying to sort out what figures would work just lead me to a rabbit hole of confusion I didn't know existed.