I would not use straight PEG. Whenever I make liquid orals I add as little PEG as necessary to dissolve the compound so say I am making 30ml total, for an oral at 50mg / ml, I tend to be able to get away with 10ml of PEG.
The trick is to heat it well in the PEG till it is fully dissolved by stirring while you heat, let it cool a bit, and add your alcohol. I never had anything crash this way.
An easier method that I have been trying out lately is to suspend it in a syrup carrier since it taste a lot better.
To answer your last question, I do not believe that PEG can alter any of the compounds your going to be using.