is this debeaker for quail
It can be used for adult quail, but the babies are very small so may be difficult to use with them.
Can they still forage ok after being debeaked.
Yes. Not as well though since you are removing the pointed end of the top beak.
Is debeaking a one time job, or will it need to be done on a regular basis like trimming their toenails?
Debeaking is not a permanent. You will need to repeat every couple months as the beak grows back.
how old is to late to debeak?
This debeaker is used for older birds (not baby chicks). There is no age limit on when to debeak your chickens.
Is there a guide with this debeaker so that one does not cut off too much?
There is a photo with instructions that shows you how much of the beak to trim.