You know you'll have to cycle the 10g before you have any fry, right? The fastest way for you to do that, probably, will be to run the 10 gallon's filter in your main tank for a while, so it can pick up some beneficial bacteria. I can't guarantee it will totally cycle the tank, but it will definitely give you a kick start.
What kind of filter are you looking at getting? Sponge filters would probably be your best bet, since Endler fry are very tiny. A small power filter could also work, but you might want to block off the intake area so the fry can't get sucked up. Covering it with pantyhose or a breeder net works, as does cutting some sponge to fit in there. Either one you do, you'll have to be sure to clean it frequently so the filter won't stop running.