I don't like replying to myself but in case someone has the same question and looks at my post I would like them to have something.
I found this thread - "https://forum.aquacomputer.de/weitere-foren/english-forum/p1428274-problems-with-lt-as-a-slave/?highlight=slave#post1428274"
It's from a person who had a 6 XT and a slave 5 LT. He's having a problem but it's not from having different board numbers.
So I guess it's OK to have this configuration.