I go 1-3/8" on all my buttons.
That puts them just shy of touching.
That spacing feels more natural to me than the 1-1/2".
Mounting them isn't really an issue if you get the nuts hand-tight, and then turn the button itself (using the prongs that hold the switch) while holding the nut in place.
You have to figure out where to start tightening, if you want the switches all oriented the same way, but it's not hard at all.