VCMP Guests HA pair with diffrent software code

Device group members (vCMP guests) must run the same version, otherwise they will not be able to failover (config sync is possible for small deviations, though).

vCMP host versions MAY differ from each other since the vcmp hosts are configured (and run) as standalone units as you noted correctly. It is not recommended though.

There are certain restrictions concerning the vcmp host/guest software compatibility which you can find in this solution guide: https://support.f5.com/csp/article/K14088

Anyway, I would suggest upgrading at least the guests to a newer version. 11.6.0 is out of support for quite a while by now - see https://support.f5.com/csp/article/K5903