Tons of plants.
Like... live, fabric, silk. TONS
Stuff the tank so you can barely see through to the other side! Trust me. You'll regret it if you don't. Low plants, mid plants and tall plants, floating plants...you name it
Also, have a few cheapy quarantines on hand. DO NOT ADD FEMALES RIGHT AWAY. they can be carrying parasites, or unseen diseases that can kill off your sorority, have you tear the tank down to sanitize, etc. Don't risk it!
Umm...Well I had a 29 long, with 4 gals, which was nice. It looked empty. But I'm finding 5-8 girls is enough
10 is alright too. Just so you can count and see each one, they can scout a good amount of territory, and generally will get along. Don't over stock
Be aware torn, chewed and ripped fins will happen, don't pull everyone out because of it... they have to sort themselves!