If confined to breeding traps for more than a couple of weeks, fish tend to become stressed. Bottom line, they can be an excellent stop-gap solution but you still want to invest in another tank to raise the fry. Bear in mind that it takes about 3 months for baby fish to get big enough to be combined with generic, non-predatory community fish.
I have only 4 guppy fry. At the mo they are in breeding net with other guppies in quarantine tank. When big guppies go in main tank would the 4 guupy fry be ok in the q. tank (25 litre). Would they produce enough ammonia to feed the bacteria?