I haven't seen anything that says you can't do this. I am running FMSA 12 and this works successfully with both 13 and 14 clients. Whilst accepting the limitations of Server 12 as compared to 13/14.
And in the past I ran FMSA 9 with 10 & 11 clients - without issues
There is always the potential that your solution will perform slower than it would in an all 13 or all 14 environment. FMP and FMS do communicate and even negotiate about who gets to do what with the intent that FMS does as much as it can. Obviously if you use a FM14 feature then FMS13 can help. There may be other subtle differences like that that will force the client to do things that server would otherwise do.
If anything I would the opposite: use FMS14 and leave the clients at 13.
My only slight reservation would be that early adoption of FMS 14 in a production environment would not be something I would not be prepared to do... immediately. FMP 14 clients, sure, but server, no, Maybe if I were solely concerned with FMS and FMP clients I might consider it, but for me Filemaker is the backbone (along with Lasso) of our on-line business and my experiences with new versions of Filemaker Server don't make for happy memories. Test in a non-production environment, then (and only then) consider bringing on line. Sorry to sound disenchanted, but I've been doing this Filemaker thing for 20 years.
There was a lot of beta testing. But there still seem to be some issue popping up with FM14. They do not seem to be widespread except for the License conflict thing. I have backups and I can survive a few crashes or go back to 13 if I need to.
Thanks for the response Wim.
All the evidence suggests that FMS14 is a big advance on FMS13 - hence deploying 14 Server should be your priority.
Well, other than not knowing it is properly licensed if using a custom SSL cert... we'd love to make deploying v14 our priority.