Welcome to Fishlore!
Congrats on the Fry! Whenever i have fry i still do my maintinance. Especially since you're feeding a lot the babies might not eat it all so the waste stays at the bottom which could lead you to an
ammonia spike. So i suggest you keep up with your waterchanges.
As far as upgrading them, you could later on. they wont grow much LOL Like Ray said after about a month you could upgrade them. If you plan on keeping the tank then i would try and go as large as possible on the tank, as mollies get rather large (some up to 4 inches!)
You can check craigslist.com or kiji or freecycle for any used aquariums. Many members have gotten alot of tanks through craigslist, as alot of people are giving up the hobby.
have fun with your fry!
:;HI1