There’s a couple ways to achieve what you want, at least as I understood your question.
As for the groups,you do not want to assign any channel to a group and to the left right main mix at the same time that group is assigned to the left right main mix. You would need to go to the channel and turn off the left right assignment. If both the channel the and group are assigned to the left right mix there is a slight phase shift due to the slight difference in latency through the group routing.
If the groups are routed only to other outputs and not the left right mix then your OK.