Your Fish
Hello em...
There's no reason to set up another tank and stress you and your fish by moving them. If you set up the main tank properly, you'll have a high fry survival rate when they arrive.
Livebearers are most comfortable with a heavily planted surface. I float Hornwort and Anacharis (Egeria densa). These are available at the chain pet stores for next to nothing and grow well close to the tank lights. This growth provides hiding places for the females and the fry.
I keep Platys, Swordtails and Guppies and have fry in the tanks all the time. I always feed all the fish a little more and a little more often. Adult fish that are plump and well fed, show little or no interest in the fry. So, I have a high fry survival rate. Probably too many, but I keep these fish in large tanks and give them plenty of clean water.
You can do the same and not have to set up separate small tanks and risk injuring your fish by trying to catch them.
B