I know this is a pretty late response, but considering nobody's given you a reply I thought I'd chime in.
I find first hit nair into regrab works on almost the entire cast. Finding the appropriate set up to hit the single hit nair in the first place has been the tough part for me.
The most common setup for single hit nair that I've found is rising fair into falling single hit nair (like double short hop fair, except with nair second). Weight and fallspeed will determine whether or not you want to tip the fair.
I personally wouldn't use non-tipped fair into regrab. Against fast fallers that are trying to approach, tipper fair into regrab works at low %. Useful against Fox, Falcon. Not very useful against any good Falco. You'll just eat a laser in the face trying to set up a falling fair.
If they're not in a position to get tipped by the fair, I would either do tomahawk or falling uair regrab. Has more conversions. Falling uair converts into rising fair, rising uair, utilt, dtilt, regrab, side b, fsmash, ftilt, rising dair.............
