Is it recommended to use a seperate DMA pair to host elements that are used for critical control functions eg. live IP video switching or routing? With the idea that this might reduce start up time for agents in a failover scenario
Yes, that's definitely a good approach which I would also highly recommend. It will indeed reduce the startup time like you mention, and separating it might also bring benefits in maintenance because updates could be done less frequently on this system while you can run the latest version on another system with all the latest features available there.
So in general, a system for critical control indeed benefits from keeping it small and separate from other dataminer systems.