All Pokemon can carry a miximum of 510 EVs, there is no exception to this. Holding an EV boosting item will not take away from that, it will only help you reach the maximum faster. And remeber, you can only put 255 (252 if you want it efficient) into one stat.
EDIT: Sorry for not getting it the first time, I think I understand now. What I think you are asking is if you get EVs on your Pokemon that arent what you are training for, will that hinder you from maxing the stat you want, and the answer is yes, if you are maxing only 1 stat then it shouldnt matter to much, but if you are maxing out 2 stats then foreign EVs (EVs that you arent training for) will stop you from maxing. This is because you can only have 510 EVs, so you have to fill that up with only EVs that you want, unless you want lower stats.