6 gallon will need about 3-4 pairs for good feeding density. Everything should be panacur-able (will kill hydroids which hurt the seahorses and eat brine shrimp) should have plants. Plastic or live for hitching, live rock that is either been pacaured or is made specifically dwarf safe. sexy shrimp are prolly not panacur safe. tank mates would be nass. snails, a few types of other snails, and hawaiin red shrimp, opae ulaa (sp?) which clean. They can be found at seahorsesource.com
Dan