I was feeding a minion in the Galaxan Wastelands when I got an error that said "your inventory is too full," even though I had the food the minion wanted in my inventory. Does that mean it would have released the minion (as in: bubble popped, win the minion) if I hadn't had a full inventory?

No it's just a standard warning, as soon as your invent is full you get that warning. You won't even be able to see the actual page of minions when you try to load the page with a full invent.

Don't worry you didn't miss out on anything :P

Ok, phew. Thanks !