Well that's not an absolute but some, including myself, would recommend a species only tank for at least a year until you are fairly certain of successfully keeping the seahorses you start with.
At the moment I don't plan on mixing species or adding other fish but some have managed it successfully. However, many more have failed in the attempt to do so.
If you fail, was it due to pathogens from the other additions or was it due to something that would have happened any way? I'd suggest be successful without first before trying others.