Clean up critters are usually added first. My recommendation for the seahorses would be to add them as your first fish. Once they are acclimated, settled in and comfortable in their surroundings then you can add tank mates depending on the size of your tank.
You did not mention the size of your tank. Unless you have a nice sized tank …. minimum of 100g, with at least 100 lbs of live rock, preferably with a refugium please do not even consider a mandarinefish. If your tank meets the above requirements then you could consider one once the tank is mature. These fish do not do well in captivity. They usually starve to death. Please read the following article before considering one of these very beautiful fish
I usually wait about 3 weeks to a month between new additions, but there are no hard fast rules I am aware of.