A 20g will work, a 29g is much better. If you do want to breed them then you should be committed to the hard work required to not just keep a spawn alive, but to ensure they become high quality, healthy and desirable by others. Unless you want to make room for 100 additional personal stock, lol...
Your fry will require heavy water changes. Some people to 100% - 200% every day. I have 2 BBS hatcheries and rotate new BBS hatches every 2 days and begin feeding the BBS within a few days of free swimming. Add other foods as they grow, etc... And then there's culling...
There are some good links in the archives and article index sections that you can read through on fry timeline and beyond... Good luck!