Will a QLXD4 receiver mount next to the SCM268 or SCM262 in the same rack space?
Sorry, the answer is no. The QLX-D rack hardware only works with other QLX-D rack hardware and cannot be cross-linked to SCM262 / 268 hardware.
It is feasible to use a rack tray to hold the QLX-D receiver and the SCM mixer. Shure has a rack tray model URT2.