Personally I tend to separate it all.

In terms of 3.75", it needs the right balance. I like the militaristic feel of GI Joe w/ slight sci-fi (Rhino & ROCC) but when it comes to Cobra, ugg. Trouble Bubbles? Pogos? HellBoars or whatever those thingys were??? I dont' mind that Cobra is a bit whacky, but there seems to be more weirdness. Personally, I loved the realism as a kid because it got my mind juices in creative mode. I'd see the F14s in movies, I'd want the toy too so I can make it do what I want. Can't say that about a Pogo.

I don't mind sci-fi elements to the storylines either. It's how they are executed (looking at the Sunbow cartoon).

Sigma6-wise, I understand that it's futuristic so I'm fine with that. And think of it this way. Would you want Sigma6 and their suits and funky vehicles to be 3.75" or would you rather have them at 8" scale so it doesn't destroy your smallverse? I tend to keep the two pretty separate.