Alright, I guess I picked a bad test example. I have three scorpions displayed with the queen hive playset. I tried a NECA stand on one of the scorpions and it was incredibly loose. But I dug the rest of the figures out of the garage and the NECA stands seem to work great with everything else.
So for anyone who's curious, the NECA stands work. The marines benefit from them and the Hive Wars aliens absolutely need them. (Those HW aliens pissed me off so much as a kid.)
Also, now that I have them in hand and can measure, the human figures stand about 4.5 inches.