...can I simply fit a string of choice - by length and gauge etc to any instrument as long as it will go on?

Yes. As long as two strings are of the same gauge, same composition, and have the same style end for attachment to the instrument, it makes no difference whether their packaging says "guitar", "banjo" or anything else.

And, in a pinch, ball-end strings can easily be converted to loop-end, and vice-versa, with a pair of pliers and a modicum of manual dexterity.