Migration to mixed 5.0 and 4.4 possible or bad idea?

Dear community.

We are preparing for a migration from a Microsoft-based Active-Directory to UCS for some time already.
Now that we could start, UCS 5.0 would be available…

As some apps are not available in 5.0 yet, is it a good idea to start with a Primary with UCS 5.0 and secondary master as app-server running 4.4?
The idea is to start with a clean 5.0 domain, so that we do not need to worry about upgrading the primary DC later.

Any feedback would be welcome.

Mixed environments are supported, but it depends on the exact applications as they must be ready for Python 3.