What decor is in the main tank?
If it's planted, or there are places to hide, I can guarantee some will survive.
I have guppies in a 10G, moderately planted tank...
I started with 4 guppies, and now I have about 15 adults and 40 fry.
(And I give guppies away to everyone I know.)
Before the tank was planted, it was gravel and a large rock as the only decor, and some fry still always survived.