Astrea and Turbo snails eat diatoms off of hard surfaces. Conches eat diatoms off of the sand bed.
I'm not sure of your tank size or type of set you have so I can't recommend number of animals or anything. But, as a rule of thumb only add a Fighting or Queen Conch if you have 2 sq ft of OPEN sand per 1" of Conch shell. I would recommend a Fighting if you go with that. They stay smaller.
For snails don't over populate like the "package deals" recommend online. Add a few at a time. If they out eat each other they will all usually die of starvation.
Diatoms are just a way of life with Reef tanks. The only real way to keep them off the glass is to use a cleaning magnet every few days or so. The snails will just prolong the time between cleanings.
Diatom explosions are almost expected in new tanks and usually go away on their own, though it doesn't hurt to add a few janitorial animals if the tank is properly cycled.
