Yes, yes and yes.
In our Notes Development Domain I have
Domino Servers running 6.5.6FP3 (1), 7.0.4FP2 (1), 8.0.2FP6, 8.5.3FP4 (30+),
9.0 (30+) all running with Domino 9.0 Directory Design.... Being
accessed by a variety of Notes Clients (8.5x and 9.x) and iNotes Users...
If you find any problems, I would be
very interested in knowing about them... I am not aware of any....
jpaganet@us.ibm.com