2). How do you plan to "mix them".
-If you mean splicing CAT 5 and CAT 3 together to form one long cable, not a good idea. It will cause impedance mismatch and reflections that raises the noise level.
-If you mean using different types between different systems/HUBs you're back to answer 1). You will have no better than a CAT 3 system. So 10 mbps will be fine, 100 mbps might work and anything above 100 mbps is out of the question.
If you do use the CAT 3 keep them on the shorter runs and away from machinery, portable phone stations and wifi transmitters.