If you are starting a new aquarium with just qa couple of fish or - in the case of fry - with a small weight/ mass of fish, there is no reason why you couldn't import most of a cycle to that tank. Gravel is not necessary, but does carry a lot of beneficial bacteria with it from an established tank. If having a clean, easily siphoned area in the fry tank is important to you, spread the gravel around the sides and back of the aquarium and leave "center stage" clear.

This still involves some time to get the sponge filters going in an established tank. 2.5s are useful for housing dropping females. I also leave a blackworm jar in there so they have something to munch on to the exclusion of fry. (For more Google GL for worm jar.)

If you are thinking of raising the fry up in there, that is very hard. Complete water changes where you put in water from a healthy tank might work if there is enough water available. The possibly 25% daily water changes in the source tank will benefit those fish too.

That does however, sound like a lot of work. It sounds unkind maybe to suggest a further expenditure of time and space, but ten-gallon tanks are cheaper than 2.5s and much better grow out tanks.