Code for how many ground wires can be under one screw on bus bar in main breaker box of residence
Ground wires are limited to 1 under the screw, unless the lug is rated for 2 it will say if it can be. as far as mixing in the main panel to me it it looks neater to mix them, not running a ground to the other side of the panel for breakers on the neutral side, also not running a neutral to the other side with breakers on the ground side. Some inspectors can get really picky on the neat and workmanship clause, although if a large enough panel is not installed to start with then rewiring would have to be done if the panel became a sub to isolate the neutrals. NEC 250.70 not more than 1,  and 110.14.A. Both say they can have more if listed / identified. (BTW there are panels that are listed for 3. Grounds under 1 lug)