Mixing brass is ok for the most part with one exception.
Some brass is a lot heavier and thicker than other brands, if you work up a maximum load in one brand of brass, and use that load in another brand with less case capacity you may get excessive pressures. Some of us load over what the loading manual recommends until we see pressure signs and then back off a grain or so of powder. Sometimes the best accuracy is with loads approacing maximum.
Norma brass is expensive, but it seems to last longer when reloading and it has more powder capacity than some of the other brands.
I also found Winchester brass to be better than Remington brass and to have more power capacity in the calibers I load.
And as an old benchrest shooter, they never mix brands of brass, always looking for another edge in the accuracy dept. Mixing brass does cause velocity variations that might not impact a hunter, but could mean the difference in a winning group or losing group for a competition shooter.