How you grip the cue doesn't matter. Index and thumb, middle and thumb, back two fingers and thumb... Doesn't matter. All that matters is it doesn't get in the way of the cue traveling level and straight.
Pressure is a personal choice. I grip the cue tighter than most. Lots prefer a light grip. Because of how I hold the cue, I can never have a light grip. The firm grip keeps the cue on line and whilst it is harder to generate lots of spin, you learn to work around it and now I have no issues with spinning the white.
What is important is not changing the pressure of the grip until the cue ball is long gone. Tightening the grip, or loosening it for that matter mid way through a shot causes the tip to go off line.
Practise keeping the pressure constant. It will help you out massively.
Pidge! I've been meaning to ask you how far do you pull the cue back when you shoot? I know you have a long bridge like me but I don't pull back to my index finger ever.