Hi,
You can keep the guppy fries in with the adults and some will survive if the tank has lot's of hiding places and cover. A grow out tank is a must if you want all the fries to live. I personally wont keep fries in a trap for more than a day but you can keep them in there for about 2 weeks if there are less than say 30 fries. Anything more than that really need more space than a plastic trap can provide.
If you mean neon tetra fries, then you wont be able to breed neons in a community tank. I suggest you read up on how to breed neon tetras since you will need a seperate tank just to breed them and get them to lay eggs. Neon tetra eggs are so small, by the time you find them they will get eaten by the other fish. you wont be able to move them in to a trap. Plus the eggs are light sensitive and wont hatch if exposed to light.