Hi-- can I use ANY mandolin strings on a mando with magnetic pickups, or do they have to be all steel. I have a spare set of bronze, but I’m not sure if the pickup will register them since only their core is steel.
Right Paul, I’d say it depends on the particular mando and pickup, and I’d use what it came with or what the maker recommends. Acoustic guitar type magnetic pickups are balanced to work with bronze wound strings, sensing only the steel cores. But most non-piezo electric mandos I’ve seen want nickel or stainless wound strings.
If your pole pieces are adjustable, I guess you could try compensating the 2 bass side courses. Just keep track of the number of turns, in case you want to put everything back. 
Yes. You can go ahead and try the bronze, but if you can’t compensate, go for nickel or steel. Those are designed for the job. What’s on it now?
Probably J67s.
http://www.juststrings.com/dad-j67.html
If this is the mandolin where you’re thinking of thinning the neck, you can use this lighter set: