I tend to use Star nails Glitters or even NSI glitters as they are fine so easy to mix.
My main thing is i love using the glitter acrylics as they are already mixed, but i know you need ones to mix with gel.
When i mix with gel, i tend to use a very thin viscosity gel to do the glitter part s you will get more glitter in the gel and get a better coverage. If you use a thick builder gel, then i've found you will not get as good a coverage as the glitter won't penetrate the gel so easily. You only need to use a thin layer of it, then obviously cap it with your normal gel after curing the glitter part.