You should be fine with the benzos. I have taken massive doses of GHB when peaking on acid before thinking that it would make the trip more manageable, but I didn't even feel it. The only thing that happened was I started to walk like I was very drunk (as happens with GHB) but mentally I didn't feel it at all. Wish I could find some acid soon, I've tripped a few times this summer (2c-i a couple times and mushies) but no acid....YET!
Oh and it should be a lot like drinking before you trip, and I do that regularly. It does diminish the effect slightly, makes it slightly less visual. If you start off a trip already drunk and in a good mood, it can be a good way to start a trip in a relaxed, low anxiety state of mind.
cheers,
-Simstim