I found a pretty good deal on a 10-gallon setup, so I bought it. The plan is to split it in three, with my two current bettas put in the two end compartments. Because Norman can't handle having a neighbor (I had them in a divided 5-gallon, and he SHREDDED his tail within a day), I will leave the center compartment fishless and just fill it with plants and decorations. Maybe an African Dwarf Frog.
That leaves my 5-gallon for the adopted betta who should be here next week (after he's had some time to de-stress in my 2.5 gallon tank).
Does that sound like a good plan? I can't think of any other fish I'd be willing to put in the center compartment since they all require more lateral swimming area than that, and I can't put a third betta in there because I don't want Norman chewing his fins down to nubs again. :-/