XLR inputs could mean that the input will accept a microphone level signal, or it may just accept a balanced line level signal, the spec sheet should indicate which.

Guitars are a different problem in that they like a high impedance input and may well not produce a line level signal, so they might need some boost. Also if run intoa lower impedance input they can loose signal stength and maybe some brightness. Again, the spec sheet should assist.

Sounds like you need a device that can mix two inputs. If you've got a model number and can find a spec sheet then I'd be happy to have a look at that to see whether it will do the job, although it sounds like the best plan would be try before you buy!

Best of luck.