If you have used glue in your t-moulding groove you haven't installed it properly

I wouldn't use lipped t-mouding if it was me as there is no need to. If anything it may create an uncomfortable edge on your CP.
3mm-6mm will be fine for the plastic if you decide to use it. Many people don't use it and instead just use a tough graphic.
If you to want to use plastic people tend to use either acrylic (Plexi etc) or polycarbonate (Lexan etc). Acrylic is much more scratch resistant than polycarbonate but some people choose to use polycarbonate as they find it easier to cut (or have at least been told it is).
Personally, I always use acrylic as I find it just as easy to cut using the right tooling. A 1/4" router bit will go though acrylic like butter.