.. gotta love the LFS
money machine. Nitrates unto themselves will not really cause algae as much as fuel their growth. Imported water impurites are moreso the cause. Silicates in your case are a major contributor to brown algaes/diatoms. Usually introduced from impure water soucres like tap water and as mentioned common with new tanks. Regular water changes with good RO
water should ease thing considerabley in short time.
Snails are always a good choice to have unless a predatory tank. Your clowns will have no objection and should be a good addition. I would suggest a few different species to add diversity. Each one will typically do better in a given area. Nassarius, cerith, trochus, turbo, nerite and margarita are good choices. About 1 snail for each 1Â½-2 gallon of tank. For your tank I would stick to smaller species like nerite, nassarius and cerith if available.
In such a small tank (26 lt?), I would not suggest an urchin. It would quickly starve and it's death would easily foul the tank. If a much larger tank it could be possible depending on species. There are several that actively hunt and can catch the "unaware" at night when sleeping.