Hey all, not quite ready to get a new betta yet, still crushed from the horrifyingly gruesome loss of my Dionysus.

But, still, I know I would one day want new little fishies to care for, so I figured I might as well start asking my questions now.
I have about six or seven spare ten gallons laying around, and I know generally a twenty is recommended for sororities, but I wondered if a group of 5-6 or so would do decent in a 10g stocked with many plants both fake and live.
If it's just a general bad idea I could go ahead and pick up a 20 later, I just figured it would be nice to just have a use to my useless tanks..lol..
Anyway, if a 10g is okay, for a tank with 5 bettas, what filter would you recommend?
And I guess any other general advice would be nice.
Part of my reasoning for wanting a sorority is well--I don't want to support our chain stores and our local aquarium store is getting worse about taking care of theirs as well. I just don't want to give them the money to buy more to abuse, you know? So I'd like to do the aquabid thing again--But I can't justify spending the shipping price on a single betta again, especially if I'm not breeding. I wouldn't mind spreading the cost around for buying multiple little gals and maybe a brother to go in his own tank. It would give me peace of mind and let me still feel okay about spending the money.
I dunno, any ideas? Is this just a bad idea, should I just wait until later when I can have a space for a 20 gallon tank? I'd love everyone's opinions and advice.