The Moderna booster is only 50ug RNA.

There is a whole bunch of trials now on mixing vaccines, but of course the number of specific combinations is very large. It does look like mixing works well, and there are some indications that mixing leads to a better response, this has been observed in particular for AZ/Biontech.

I got three different vaccines, following the recommendations at the time. So at least here in Germany we do freely mix them.