Right - just get non sudsing ammonia with no fragrence added. The dollar store usually has regular ammonia and lemon scented, so you would want the regular. Just add some to the new tank each day to build up the bacteria that you'll already have started on the filter that you seeded. When you get the frogs you'll do a large water change, add fresh water treated with dechlor or Prime, then add the frogs.
Bettas aren't particularly messy fish either, which is why I assumed one frog would be about the same as one betta. I've never kept frogs tho. Perhaps someone else who's kept both will chime in with some info on that.