Hi Steven91,
I do not see a problem with using your 90 gallon FW tank and converting it into a SW tank for seahorses. When it comes to seahorses Dan Underwood at Seahorse Source says that there should be 30 gallons for each pair of ponies. So for a 90 gallon tank you could keep 6 seahorses.
Seahorses are vastly different in the way they are kept as compared to other SW fish due to their susceptibility of getting bacterial infections. THEY MUST HAVE EXCEPTIONAL WATER QUALITY TO REMAIN HEALTHY. Very good water quality is maintained with frequent large water changes, an oversized skimmer, 10Xs or more turnover rate through the filtration and diligent over the top husbandry. For example: I have 3 seahorses in a 50 gallon system and I do 24 gallons a week in water changes. If you are keeping 6 seahorses, you may be doing over 40 gallons a week depending on how you feed, the filtration ect... It is costly and can be back breaking if your water change method is not well thought out. If you were willing to keep less seahorses, then smaller WCs could be done.
I have been keeping erectus seahorses for over 4 years. I have tried keeping them with docile fish and with a sandbed. For me, I decided my ponies stay healthier in a species only tank with a painted bare bottom (to look like sand). I do have live rock, some hardy soft coral and macro algae so the tank is still natural looking.
These are just some things to consider. I recommend doing lots of research before you make the switch. This topic has lots of good information in the stickies that should help you. There are a number of us who will be happy to answer questions too.