5 gallons for less than 60 or so dwarves, in my opinion, is still way too large. I'd go with a maximum of a 1 gallon tank if you are going to order a dozen seahorses (and I still think this is pretty big ~ a half-gallon would be better). The tank will need daily water care maintenance too because the brine shrimp will foul the water. You can buy brine shrimp eggs from
www.brineshrimpdirect.com. I've been buying the 1 lb. can of premium grade (90% hatch) rate eggs and I've been pleased with them. Ideally you should decapsulate the brine shrimp eggs too, which will result in a much higher nutritional value (at least for the first 12 hours).
I hate to repeat myself, but it's a lot of labor! Have you considered any of the larger species that don't have such demanding nutritional needs? Many of the larger species can adapt to eating frozen mysis, cyclop-eeze, etc.