I'd get a whole kit for $40 that comes with tank, filter and hood. It'll be worth it considering hoods themselves cost $30, sometimes more.
I have a 10 gallon with 5 females, had 7, but 2 became sick and died :( I added them all at once. Some say add them 15 minutes apart, but I figured they were in the cups with each other and could see each other, so they might as well just be together. Now some may not agree with me here, but when I put all my girls in I watched them for a minute, then turned the lights off and didn't look at them until the morning. If you're like me and love your girls, you're going to get stressed seeing them fight and sort out the pecking order and seeing ripped fins. But its best to just not let it get to you, and to let them sort it out.
Also make sure you make the tank HEAVILY planted with lots of caves. The girls love it like that, and it really does decrease the aggression when one girl decides to go on a rampage, and the others can hide. Good luck!