There are no rules really but most people will generally stick to one type for the entire bass. I can think of a few examples of some pros that mixed different types. Johnny Colt (former Black Crowes) used to use Rotosound rounds on the E and A and Rotosound flats on the D and G. T Bone Wolk (Hall and Oates) used a D'Addario round on the E and the rest were halfrounds. And finally, current Metallica bassist has one of the strangest signature sets of Dunlop strings where the E through G strings are stainless steel rounds (and the E, A, and D are tapered at that) and the B string is a full non tapered nickelplated round! Very unusual.