i'm trying to figure out how i want to base things.

i think my rangers, wraithguard, and swooping hawks are probably going to go three to a stand in the current rectangle stands.

striking scorpions and howling banshees will probably go in square stands.

my guardians will probably be mixed on a formation-by-formation basis, i guess. just because i only have 24 square stands...

It's definitely a legacy thing. The old models fit ok (Squats have no problems with square bases). All of the current models and generally any designed for rectangular bases have trouble with the square ones. High strength glue (Plastic welder, a lot like fast evaporating water) can also soften the bases enough to get some models in but don't try it with the new Eldar sprues. I've warped a few bases trying to fit them in and had to rebase them.