Mixing depressants is always a risky game.
Clozapines sedation is though antagonization of the histamine receptor, while it will be potent, i believe sedation through this mechanism is one of the safest with regards respiratory complications.
Of you have been taking clozapine for a while and are well tolerated to its effecys then it should be safe to add a SMALL dose of a depressant like benzos.
Id avoid alcohol as its just very unpredictable and causes alot of depressant ODs.
Just please be aware that when it comes to cnd depressants, 1+1 rarely equals 2, it more like 1+1=4.
Start small and go slow.