+1 to what Olympia has said, but I would say get 5 females minimum, 6 max, and no other fish. 6 bettas stock a 10 gallon to the point where another school of fish, such as tetras, would be too much (and yes, I am a massive hypocrite, but my tank is triple filtered, heavily planted and I do daily water changes, so don't copy my bad example. :p).
There are also many other plants that are easy to keep - java fern, java moss, anubias, anacharis, ambulia, cabomba (if it's legal where you are, it isn't here), hornwort, lacefern, rotala and ludwigia are all some examples. Floating plants like duckweed and water lettuce will make your bettas feel more secure. :)
As far as planting, stuff that tank full. The more plants you have, breaking up the lines of sight, the more likely your sorority is to be peaceful.