@tradz I run nicehash and also GPU mine my plots without issue. If you find the mining is lagging a little, use task manager to set it's priority to above normal, and it should be a bit faster.
As for trying to mine on nicehash and another coin, you can - but you'll be splitting the computational power and adding overhead. So instead of 100% of GPU going to nicehash, you'll be getting something like 45% to Nicehash, 45% to coin X and 10% lost to overhead.