MIxing brake fluids is no big deal, especially if you mix DOT 3 with DOT 3.
You need to check the bottle but most DOT 4 can be mixed with DOT 3 and vice versa as well.
DOT 5 is the problem, I believe it's silicone based and will not mix with conventional brake fluid DOT 3/DOT 4.
I believe DOT 4 is a synthetic version of DOT 3.
Again, when in doubt read the label.
AP 600 and silicone based fluids do not mix with others. Read the cans. They should have some warning if they are not compatible with other fluids. You will mix them eventually when you bleed brakes, as there will probably be some left in system somewhere.