In a 34 gallon you could go with a pair, but I wouldn't push more than that.
Erectus are what I keep/raise and are tons of fun. Especially if you want to try breeding. Southern Erectus are the easiest to breed and loads of enjoyment!
Mandarins are great tank mates as long as you can feed them. I have one myself but he eats frozen food, so no worries as far as the pod load goes. Seahorses will quickly wipe out a pod population just grazing throughout the day. Just something to keep in mind.
You might want to see what temp your tank runs at before adding the seahorses. Lots of people have had good luck lowering the temp of plug in plays by swapping out the pump, but I don't have much experience in that department.
As far as keeping them healthy temp is number one.
Get them from a reliable source that sells captive brad not tank raised. The two are very different. Dan and Abbie at seahorsesource.com are awesome. I would definitely suggest checking them out!
One other thing that helps is rinsing the frozen mysis. Allow the mysis to thaw in a bit of saltwater then rinse in a net and add to fresh saltwater. That really cuts down on the amount of nutrients that get added to the tank and possibility of making the seahorses sick.
Some good meds to have on hand are neomycin, triple sulfa, and diamox. Diamox is a prescription so you have to get from a vet or here:
http://www.inhousedrugstore.com/neurological/diamox.html
Read, read, read, and ask questions!
