Changing cassette, can I change ratios
This would be completely fine. You only really have to worry when going to a much wider range. Smaller road derailleurs cannot deal well with big differences between the largest and smallest gear, but they can all handle a 15 tooth difference. Things only start to get compatible once you move beyond 11-28.  Shimano/SRAM are compatible for 8-9-10 speed cassettes. Things aren't compatible at the 11 speed level. According to this chart, Shimano and SRAM mountain Cassettes are compatible at 11 speed, but for the road there is a small difference in sprocket pitch.