My tank has room for 3... is this a good number to get? or should they be kept in a larger numbers to really be happy. Should I get one male and 2 females... all males... all females... or is it not possible to sex them anyway...
I would say that if your tank has room for three, and thats the type of fish you want, then get them. I would say 3 cories as an absolute MINIMUM, because they really do like to be in groups. Three will be ok though, i have six. Two bronze, two julii and two albino. The bronze ones have already bred after 24 hours of being in the tank! With the male and female situation, if you can sex them at the shop, id say go for 1 male 2 females, all females, or all males. The shop will probably stock them at a smaller stage in their lives, where you wont really be able to sex them. just get three cories in this case i would say.
Do you hope to breed them some day? If so, try to get one female and two males. This is so that they have the best chance to fertilize as many eggs as possible. If not, gender won't matter.
You might have a hard time picking out males and females if your lfs stocks them very young, but here's how I do it. First I pick the biggest healthy looking fish in the tank. That one will usually be a female. Once it's in the bag (or if they will put it into one of those hard plastic containers before putting it in the bag, so much the better) I look down on it to get a good idea of it's profile from the top. Then I look into the tank and try to find the males. They will be much slimmer. With peppered corys, the males might be shorter too, but don't just pick the smallest ones, look for the thin ones.
And do try to get as many of them as you have room for. They are schooling fish that actually interact with each other and you will enjoy watching them. Please let us know what you get. I'd love to see pictures too, if you can post them.