I would add them as you see fit. Do you have a lot of algea in the tank? Are there any problems that bringing in a clean up crew is required? I would suggest adding some snails, maybe 1 per 5 gallons, or something like that. Add enough for 1/2 of the tank, if they aren't maintaining it properly, add more later. There is no sense for you to bring in a ridiculous number of new critters into your system. It might throw your cycle off whack, since it's a young tank. As I've heard, I probably wouldn't even add cleaners unless you're trying to solve an issue.
Start with a small, hardy fish for your tank, possibly some snails. You want to add your smallest fish first, biggest fish should be the one you add last so that the territories work out. Corals, add later. If you're gonna add any clean up crew, add snails. Crabs will attack other critters as well, so you don't want too many otherwise they'll fight. You want to balance things out.
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Yamaha TY250