Error placing clock pin with Clock Control Block

I think only one signal can get onto the altclkcttrl block from the fabric, and it is selected statically(i.e. at configuration), rather than dynamically. The Handbook should have a section explicitly showing the altclkctrl muxes. What you're doing is fine, but you want to be careful not to switch between clocks in such a way that you get a clock glitch, which is very easy to do.