Sexing mollies and the like
 

I am looking to buy some mollies or swordtails and I want them to breed. How to I tell the diff between male and female? Do they breed fast? What size tank do I need? I have an extra 25g tank. Is this big enough?

They Breed like rabbits.
Yes, the 25gal will suffice for breeding and as a grow-out tank for the fry.
What do you intend for the 100's of fry/juvenile fish you'll be breeding?

Both of these species are livebearers and in both the males and females are easily told apart. The females will have a normal fan shaped anal fin, the males will have a modified anal fin called a gonopodium.
This gonopodium has various hooks and spines to help grasp the female during mating. One look and you'll see what I'm talking about.
Do a search on livebearers or gonopodium.

I intend to raise the fry in a 30g tank (the raising tank often used to raise my clownfish fry and stuff). I recently transferred it back to freshwater. this tank serves as a raising tank for all my fish. I see what you are talking about and I am purchasing 4 tommorow. I sell and trade fish and the like with friends. Some recently asked me if I bred FW and I said no but then told them give me a month and I'll have some FW fish for you. So I am buying 4 Swordtails and in a different tank I am breeding Gouramis. So I will beef up my livestock and broaden my horizon to a new type of aquarium keeping.

swordtails are even easier to tell apart cause the males have a sabre ( the long tail) and the females dont and the females are a bit fatter.
but dont go by that alone, cause i bought 2 males and 2 females, but it turned out that one of the females was actually a late blooming male, his tail just took longer to grow, so now he has the body of a female with a tail and a gonopodium
so just look for the gonopodium

Problem is, late blooming male swordtails have a female type anal fin, not a gonopodium, as do all immature livebearers :(

Only time tells.

I read somewhere that some female swordtails will actually turn male so that might be what you have, not a late bloomer.