As Sam alludes to, the combination of running WAS v6 and v7 on RHEL would have never been tested.
As you probably know, WAS v7 went End of Support 30/04/2018 and WAS v6.1 30/09/2013.
It might work OK, it might not, who knows? What I do know is that you will be on your own if you encounter any issues.
------------------------------
Bob O'Hanlon
Consultant
IBM
Winchester
------------------------------