So Ive got a 20g with only a snail and a swamp darter and Id like to get some females in there. The swamp darter is maybe an inch and a half long and just sits under the plant in the back corner and Ive only seen him roam like 3 times in the like month Ive had him so I dont think they will notice him much less him be a problem.
How many females should I add at a time and how many total?
You'll want a minimum of about six, though you could probably have something like ten in there if you wanted. When you get them, after a month of quarentine and letting them see each other so you have an idea of agression levels you can either introduce them all at a same time (not good for your cycle) or you could introduce the meekest first, and slowly add one at a time until you get to the most agressive. Just make sure you have a ton of hiding spots and a hospital tank on hand should a girl get too agressive or too hurt.
That would be fine, but I don't think that you should put too many in. I think that 5- 10 would be the maximum. Once you put them in though, you should do some watching to make sure the weaker and smaller ones don't get picked on by the most aggressive ones.