Here's a good thread that talks about betta sorority basics. http://www.bettafish.com/showthread.php?t=29402
I agree, lots of hiding spots, i.e. lots of plants. No fewer than 4 females is recommended. In a 20 gallon you can safely have 9-10. I have 9 in a 20gallon. I love my sorority. its very rewarding. Female bettas are so cute and fun
Look forward to seeing your plans!