First of all, every fish addition decreases the chances of a successful keeping of seahorses.
Seahorses for some reason, are very susceptible to pathogens they haven't been exposed to when rearing.
As most fish you add to the tank don't come from the same source, they are being exposed possibly to new pathogens each time you add a fish.
Next, seahorses don't LIKE low flow but rather enjoy having low medium and even high flow areas as long as there are hitches for them in each area so they can choose which area when they want. Some even play in the output flow of power heads. Main thing is that there is no place where flow can direct them against an object so they can be injured.
At the bottom of
http://www.angelfire.com/ab/rayjay/seahorsekeeping.html there are a bunch of links helpful to someone starting seahorse keeping and one of the links is to the "org's" Tankmates Guide.
If you do want to have fish other than seahorses in the tank then the guide can be helpful to you. There are multiple reasons for the ratings on each of the fish, but zero is obviously the best and one rating can be chancy.
It's also best that you keep the seahorses only for the first year or so, making it easier to diagnose reasons for problems that develop.
It's important to note that following all the recommendations won't guarantee success in the seahorse keeping hobby, and indeed, there are successes out there that completely ignore some or all of the advice, but the odds are much more favorable when abiding by them.
