I'll take a stab, since nobody else has piped up.
1. Personally, I like codium ... it grows slowly and I've had luck with it. Just started trying some helimeda. Any macro you can grow that won't require persistent pruning should be fine.
2. I've had gorgs, but the horses didn't hitch to them ... not sure the gorgs would have liked it anyway. I mostly use peices of branching rock. Mangoves would be great if you have experience with plants. Airlines seem to be sh favorites.
3. Erectus seem to have the reputation as being the hardiest ... perhaps due to the fact more is known about them and that they are more readily available from top breeders. Color shmolor. Seahorses are incredible creatures ... their activities, personalities, manorisms, interactions, etc. are what make them so, not color. My favorite seahorse was a bland colored female reidi. When I approached the tank, she met me at the front glass, swimming back and forth like a puppy who was glad to see me. If I put my hand in the water, she would instantly hitch to my finger. I wouldn't have traded her for a hundred fire-engine red horses. Unfortueantly she died at only 2 1/2 years old.
4. The rule of thumb I always heard was 3x-6x tank volume turn over. Personally, I think this is a bit low. I shoot for a fairly even flow throughout the tank (no dead spots). To acheive this in a tall tank, I found it takes 6x-8x. I put all my flow in at the top though, so by the time it reaches the bottom of a tall tank it is very gentle.
5. For a beginner, I would stay with a species tank. Seahorses are challenging enough. Adding anything else adds some degree of risk, and I would try to minimize risk at first. That said, if you do add tank mates, stay away from fast or perpetual swimmers. Stay away from potential bullies, including all clowns and damsels. Also, stay away from all other species of seahorse and pipefish, as there is an increased desease risk. Also, I don't include anything with hands in my cleanup crew, but I might be overly protective there.
Overall advice: don't try to do too much right from the start. Everyone dreams of the perfect seahorse environment, with natural hitches, flowing macros, etc. My advice would be to concentrate on the seahorse needs first, then after having sucess there, gradually add other things. I call it the KISS method (Keep It Simple Stupid).