As long as the file has an extension of .fmp12 the fles will be usable/hostable.
Thank you very much!
Snow Leopard is pretty long in the tooth and is stuck on Java 6 with its vulnerabilities. If you upgrade, I would certainly look to Mountain Lion for a server. Snow Leopard is also being deprecated by FileMaker, so don't count on future versions supporting Snow Leopard. Lastly, Snow Leopard Server is very expensive ($500) and very cheap on Mountain Lion ($29). So it is a no brainer for expenses.
But yes, all of the FMP 12 client applications will work with all FMS 12 versions. So FMP 12.0.4 will open files on FMS 12.0.3. But I would encourage you to upgrade your FMS to 12.0.4 since it will get you completely off of Java 6 and onto Java 7. The upgrade is free, but not as easy as most upgrades because you have to completely uninstall FMS 12.0.3 and then reinstall FMS 12.0.4. But the extra work is worth getting away from the security risks on Java 6.