Honestly, that sort of thing might (emphasis might) be realistic for future combat, but I can't see it being appropriate for UFO:AI. It would detract from gameplay by either reducing the importance of human soldiers or cluttering up the battlescape with 8-10 of the little bastards. It won't get my vote.
Well the smaller size just means they could fit through doorways, not that you should be able to spam them. Like any UGV, there's only so many that you should be allowed to fit in the UGV pods on the dropship.
As for competing with soldiers, that seems like an issue already on the table, since the Ares and Phoenix (and later, more advanced versions) already compete with snipers and support infantry that fight in the open. Smaller armed UGVs would simply compete with (or complement, depending on balance) CQC focused soldiers. So it seems like the differences between human and machine combatants would have to lie elsewhere of a size difference.
This would just buy you the ability to have something more expendable to spearhead CQ raids on buildings and UFOs, plus something to do with your UGV pod space if your current mission is in a cramped, urban situation.
I don't know what's in SVN, but I know for a fact that Sitters modelled wheeled, tracked and hover versions of the Ares specifically. If I'd meant the Phoenix I would've said so.
Yes, that is how the Phoenix was planned.
Do you folks have a wishlist of all the UGV's you'd eventually like to have, plus any descriptions of them?
For example I heard at some point that you had plans for advanced UGVs, partially based on alien technology, for the mid to late game.