Hi, each major version that 3CX releases we make heavy improvements and also add new feature requests where in this case we always recommend that if you want to have a bridge between 2 PBX and a reliable connection it is better to use the same version.
As far if you want to connect a V12.5 you can but you will have some problems in SIP and also in Presence information as we revamped this area.
If you want you can update your V12.5 to a V15. But if you want and you are happy its up to you.
Why it takes 30 mins to connect is due that in V12.5 re-connection will be triggered after 30 mins.
In V14 and V15 this is changed to 1 min.
As far how testing team tests major versions we check all supported versions (both bridges and clients) but in V14 and V15, structural changes where made in server side so I always suggest that your system is up to date.
Thanks