[rayjay]

IMO, it's not worth the risk of potential problems in tying a seahorse tank to a reef tank.
First there is the potential to expose the seahorses to pathogens they haven't been raised with and are VERY susceptible to, but also the seahorse tank should be kept in the range of 68° to 74° F to help mitigate bacterial problems.
The minimum recommended tank size for ONE PAIR of standard sized seahorses is 30g.
As for rock, sand, decor etc, that all depends on the needs/desires of the hobbyist themselves. I use rock as my biological filter with most of it in the sump so I can see the seahorses easier, and, I have bare bottom merely for the ease of cleaning. For decor I use artificial plant replications that don't have higher light needs that many live plants do.
As for adding other fish to the tank, first you would need increased volume, but also have to accept the possibility that pathogen introduction from the other fish may be problematic.
Some people have great success with other fish, but MANY end up with a lot of problems.