Is there a multiband compressor plugin available that I can put on a subgroup (for example) to get certain frequency peaks under control when I have a group singing together?
When you buy a brand new SQ, there will be no Multiband Compressor on board. However you can purchase a variety of plugins called Add-Ons. Here is the link: https://shop.allen-heath.com/index.php?route=product/category&path=78_81Add ons for SQ
which eq do you recommend using to do what I’m thinking?
Are you asking which plugin, or which frequencies? Generally speaking, the MultiBD multiband compressor handles broad areas and the DynEQ4 dynamic EQ is more targeted.
A&H also sells bundles of these plugins for the SQ series. You can get the “Dynamics” bundle that includes both the DynEQ and MultiBD as well as a de-esser for $149 which is less than buying the DynEQ and MultiBD separately.
https://shop.allen-heath.com/index.php?route=product/category&path=78_82