Hi!
Your molly momma looks an odd shape of pregnancy to me. Usually they grow the bellies forward, and then square out. But your momma looks like she went down, to me that says she could be full of eggs.
I would let her back into the larger tank, and add the plants in a corner in that tank. You may want to invest in a breeders trap or net, $10 at most and definitely worth it if you cant buy a whole nother tank