Hi Jane I can't see that synthetic hair matters, just that it wants to be a soft brush that is less likely to leave brushmarks in the paint.
I've not used the brand of glass paint you mention, but if you've stirred it well then it must be that it's very transparent and requires several coats. Though I'm surprised. The Pebeo website says you can apply two coats of color without first baking the initial coat.
Mixing the paints -- I don't see why not, as they surely are all fundamentally the same thing. I would try and see what happens, perhaps painting the mixture on a "sacrifice" item so it doesn't matter if it doesn't turn out well. I wonder if it takes a lot to get the colors to mix well, rather than be streaky?
