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You could just go the bad ass way and just forget about scales and theory and what not. or you could learn theory. There are formulas for all the different types of scales, and this will apply to all instruments, mandolin, banjo, guitar, ukulele etc. just pick up a theory book and learn from that. there is a book out there called "mandolin scales and studies" you could use that as a refrence to the theory, but scales wont help you unless you know the theory behind it. as for solos you just have to go with it, you know, just jam and find something that works, scales cant really teach you to solo, they are just guidlines.
sorry if im preaching