Client has RB4011iGS router running ROS v6 but starting to get more access points so thought about CAPsMAN but some of those are on ROS v7. Can a ROS v6 router running CAPsMAN managed ROS v7 access points?

Supplemental question but can the RB4011iGS run ROS v7 okay? I've been reluctant to upgrade it as the site is pretty much 7am-midnight usage.

The question mentioned in the title is something else then the question in your post and not really clear.

Can a router running ROS6 manage capsman for ROS7 devices ? -->
If you mean with ROS7 devices, AX-devices or AC-devices using wave2 drivers, a firm NO.
If you mean ROS7 devices still using legacy wifi, yes.

Other question from your post: can rb4011 run ROS7 ? Yes it can. No problem.

But ... can RB4011 run wifiwave2 drivers ? Yes and no. For 5GHz yes but then you loose 2GHz radio.
You can stay with RB4011 on legacy wifi drivers (wireless package) and then you can keep using both.

So you need to be careful if you want to use RB4011 as controller for an environment having partially or all wave2 devices.
See also here:
https://help.mikrotik.com/docs/display/ROS/Wireless

I've updated the post to make it clear. There were two questions in there but you've answered them both. Thanks.