Nothing bad, for sure. They will share your cpu, and mine... Total mining speed will be little slower rather then running 1 miner, so generally it's not a good idea

Are we talking only about GPU mining or node mining also ? If so i can recommend very interesting website with comparison https://masternodes.pro/

overwise whattomine will solve all your issues

Why everyone here say that it's not profitable? It;s awesome, i am mining for a year right now, and it's cool. Don't need any other hot source to make my flat warm and coooooozy 

i wonder if gpustats could be integrated with claymore miner, so we have really good statiostics about which models what can achieve... probably it is possible

Is there any info about ethereum hashrate ? Want to add one more into my green eth rig to make it full  

in depends on your skills and probabilities. Of you have free/low cose electricity, special place for a rig and are ok with some configuring, then - why not?