If it is a psychedelic RC, cross tolerance is a definite possibility - but without specifics, it is difficult to be more clear than that.
So...rule of thumb is to wait for a few days (3-5; but the more the better).
No point wasting good LSD (assuming it is) because you werent patient enough to wait it out.
As for LSD + ambien; that sounds like a recipe for delerium and potential disaster, from what i've read about that stuff anyway, ive never taken it.
Redosing LSD has a bit of a fine margin for error IME. After a couple of hours it's just a complete waste - but when it does work, it is likely to extend the length of a trip rather than intensify its effects. If you have issues with tolerance, redosing wont do anything to help that.