There is no direct way to disable the 5GHz radio (or 2.4GHz radio) from AP groups. However, you can create a "WLAN Group" and uncheck all the WLANs in that group. Next, go to the AP group and select the WLAN Group under Radio Settings :: Radio B/G/N (5 GHz). This will effectively disable all the WLANs in 5 GHz radio for all those APs that are part of that AP group.
For AP channel, we have channelfly that chooses the best channel for each AP automatically