You should be fine - just don't expect to be running full HD videos with complex Quartz Composition overlays and 8 effects at once in the FX Sequencer :) The best thing to do, if you do have problems (low frame rates, etc), would be to reduce the dimensions of your external display, and make sure that MixEmergency's windows don't overlap any moving parts of Scratch Live/Serato DJ (e.g. the waveforms, virtual decks, etc), as these can put a lot more stress on the graphics system.