Hi Teru,
Yes, you can use QoS config on the the interface directly without creating a map-class frame-relay and applying the frame-relay class. But in that case the QoS config will be applied to all the frame-relay sub-interfaces you have under the physical interface where you applied the QoS. So, you lose flexibilty in doing such.
When using map-class each frame-relay sub-interfaces and PVC can be assigned to different QoS config and that is helpful when you ahve different sites with different QoS requirements.
Random-detect and Custom-queue-list both can be used in interface level, but at the same time either one of them can be used.
Router(config-if)#random-detect
Must remove custom-queue configuration first.
Router(config-if)#no custom-queue-list 1
Router(config-if)#random-detect
Router(config-if)#custom-queue-list 1
Must remove RED configuration first.
Hope this helps !!
Thanks.