no issues mixing them. just remmeber that if u are installing them urself, dimarzio hot lead is RED while duncan's is BLACK. some ppl get those confused.
1. when each humbucker is selected, they dont affect each other. when switch is in middle position (assuming 3 way switch). both pups are on at the same time. and whether or not they mix well is up to personal preference.
2. when u switch from bridge to neck, some ppl like them to sound in the same ballpark (neck generally warmer sounding than bridge). or some like a total change of tone. up to preference as well.
and yes in general, bridge pups have higher output than necks due to string resonance. sting vibrates less at the bridge and more at the neck area therefore bridge pups have to be of high output to balance the sound when u play.
however u can always get 2 similar pups and put the in neck and bridge. just lower the neck one. a nice combo i've seen ppl use is dual Duncan JBs.