Generally if a clip on unit will line up well enough to see through it's good to go. Sometimes certain mounts are lower or higher and my PVS 30 still works fine.
I don't fully follow you though, "drop in" rail system (like non-FF) or a "drop in the" rail system (like a PRI unit)?
If it wiggles, and it's a collimated sight, it won't matter. The NV will wobble a bit I guess but the scope which does all the work won't. I may not be ideal.
If it's a drop IN the rail system, then like I said, as long as it lines up "good enough" it'll work. What's good enough? Stick it on and make that determination for yourself.