Resolution mis-match is ok. Each monitor will retain its native resolution. What could hamstring you is if one of those monitors has a greater refresh rate than 60Hz, it won't be realized. The GPU can only output a single refresh rate and the lower monitor will of course be limited to 60Hz which kills any high refresh rate benefit you may have gotten from the fast monitor. Only way to get around that is to make sure both monitors have the same refresh rate capability.
I use a 24" 1920x1080 @ 60Hz and a 19" 1280x1024 @ 60Hz.