2.5G although fish fry are small a 10G , and usually more like a 20G is better.
Reasons being due to having to feed frequently like 3-5 times a day pollutes water fast and can ruin water quality in a 2.5G whereas, a 10-20G gives you more room for growth and more time for doing big water changes since you have more water volume.
Fish can produce hormones, and while growing with a minimal amount of water it can stunt growth. Opposite of what you are striving for.
The instant cycle above is a perfect thing to do. If you have a filter pad you can hang in a cycled tank under the outflow of a HOB
/ hang on back filter. Grab it out and use it when you need it. I used to have a bag (from the produce department which held Onions, garlic or lemons) which held a big bag of ceramic filter media just for any old group of fish I might end up with!
New to FW fish? First tank in a long time or EVER??? Need some info on keeping fish?
Start here - Aquarium Advice Article
FW Favorites right now Clown Loaches, any shrimp - SW Vase Red Mangroves