First off: Omedeto on getting these guys.
Something I keep forgetting to do: check the fish index. The members on here put a lot of work into it, and it makes for an easy way of getting answers.
However, since you have already posted the questions...
1. I, personally, just go by girth, though that may not be the best way to do it at an lfs. However, the bigger they are (length) and the fatter, the higher the chances of them being females. I believe there was also something regarding lines and colors, but that has long since been forgotten...
2. Unintentional breeding is done easily enough. If you have a decent number of females and males, the females will get gravid, and start dropping eggs usually at first light at dawn, or from what The-Wolf has said, also at dusk. The eggs might get eaten, but some make it to the substrate, so when you do a water change, let the bucket of old tank water sit with an airstone overnight and you might see little black lines--the fry.
Intentional: you could line the tank bottom with marbles or get a net and drape it over the tank sides, dipping it in the middle so there's room for the potential parents to spawn. this way, the eggs go through the net or under the marbles, where the parents cannot get to them.
3. I'd say at least 7 of these guys in a tank, since they are shoaling fish and feel safest in large groups. However, the size of the tank can also be an issue, so would it be possible for you to post that? (can't see your sig in "add reply" if the size is there *shrugs*)
