Some comments:
1. The fighting conch will die in such a small tank.
2. Hermits are omnivourous scavengers. If you put in a lot, they will find and eat most of your seahorse food before the horses do. If you really want hermits, I would not put in more than two.
3. Depending on the snail, I would start with a small amount and ramp up if needed. For astrea sized, maybe a half dozen. For smaller snails like stomatella, you could add a dozen or so and they will breed to the proper levels in short order.
4. To add some detail to pledosophys comment on shrimp, horses do not like to be cleaned.
5. If you are going to use macro algae, it will use a lot of nitrogen and phosphate making a dandy filter... if you have enough in the tank.
Final comment. 20 gallons is on the small side for seahorses. If you are the sort of person who is good with regular water changes and maintenance, you may be ok. If you are like me, you need a bigger tank like a 30 gallon.

Water quality is usually an issue in such a small tank.
Good luck.
Fred