Just remember that breeding takes a lot of space and is unlikely to make much money. Since you are expecting to be given adult fish for free, how much do you expect others will pay you for the fry?
Even the humble guppy needs grow-out space, and you get a new batch every month which will need new space. Fry need to be a few months old before they can be found new homes. Swordtails and danios need room to manoeuvre, and swordtail fry need quite a bit of growing out space. Also, all these eat their fry/eggs, so you need separate fry tanks. Kribs look after their babies but get very aggressive with other fish when spawning so would need a tank to themselves.
To sum up, breeding isn't something to do with the spare few inches in one of your 10 gallon tanks.
Breeding is a great experience and I wouldn't dissuade you, but if you want to make easy money you're probably better off delivering newspaper rounds. There are people in the hobby who manage to cover their costs and even make a moderate income from breeding, but they are usually the people who could afford a certain amount of financial outlay at the beginning, to set up a proper fishroom and to buy good breeding stock.
If it's the experience of bringing up babies you are after, then I would suggest platies or guppies, or possibly cherry barbs, since I seem to remember you only have 10 gallon tanks. You will need to be very careful about water quality, since fry are more sensitive than adults.