they are the fighter fish. and if a male betta was in with a lot of females, i hear that he would work himself dead tired with chasing the girls and building nests and etc... Its recommended to only keep one male betta alone (not with a female unless breeding). you can put a male betta with other fish like the cleaner fish, a snail, some tetras (someone else would have more info on this for you). females you can keep in a sorority, like 4-5+ together in a large enough tank without any problems. some people don't have a problem keeping two or three together but it all depends on the fish's personality.
from other things i have read, if all your baby bettas are raised together with the father still in the tank (acting as an alpha fish) people have had success in keeping males together (BUT they never separate them for even 5 minutes because thats when they become territorial and start fighting). So, a 2 gallon is preferred to keep a single betta in, male or female, with maybe a snail or something. 5+ gallons you could keep 3 females in together if they got along (which is unlikely unless they all have calm, peaceful temperaments...my 3 girls go at it if i put them together, so I would have to have more then 3 in there together).
As for picking out a betta, personal preference as to what you like to look at pretty much. Checking them over, make sure there is no finrot or other visible illnesses, unless you plan on nursing it back to health if needed and giving it a good home. (when I buy from a pet store, petsmart, petco, walmart, I don't worry about it because I will nurse it back to health and give it a good home).
does that help any?
bettas also like warm, clean water. so you will need a heater and thermometer in the tank if it gets cold where your at. i keep my tanks between 78 and 82 degrees F. and keeping it clean, means partial or complete water changes and making sure the PH and all that is ok, either using bubble buddies (what i use to remove chlorine, and like cycle to help with the bacteria) and test strips to test the waters hardness and alkalinity and ph and such.