It's purely personal taste but I think mixing them would look nice. I personally think that all matching sofas and chairs look a bit naff, so imo - yes, it would be fine to mix leathe and fabric. Have a flick through some housey magazines next time you're in the supermarket to get ideas!
I presently have a brown leather 3 and 2 seater sofa's I am looking at changing the leather 3 seater to a material 2 seater to match the leather 2 seater that I have. which means I would have a 2 seater brown leather sofa and a 2 seater wheat coloured material sofa, just not sure if I can have leather and material together Would appreciate opinions from others