If the tap is ahead of the service disconnecting means then it is service entrance conductors...right.They do not belong in the same conduit as non service conductors.
But if the feeder is a tap from the existing feeder...I don't know of a restriction of feeder taps with other circuits?
Also if the service has a 800amp main and 400amps of solar on the backfeed the bus in the MDP must be rated for minimum 1000amps.
2011 NEC 705.12(D)(2) 120% rule:
1000amp bus x 120% = 1200amp
800ampmain = 400amp solar = 1200amp
