Really excellent advices, mate. Thanks mucho.

As for not cut or boost more than 3dB, it is true that over doing on EQ can lead to sounds with no character and definitions, so finding sound which will fit with almost no touch on EQ will be better choice.

Voxengo SPAN is my good helpful friend and I always use it on master bus. :) together with sonalksis freeG.
Ears are the best judge. It is very true. If it sounds good it is good.

With peaks, I have got mainly problems with kicks and snare sounds,. They use to have have very hi peaks in their fundamental notes.
Kicks can be compressed a bit, and same with sanres, but now I just have idea, that snares could be treated by moving its start from fundamental to 1st harmonic. hmm. but maybe it will sound weak,. i must try. or maybe not move its start, but just EQ the fundamental a few dBs down...

dodgy: A/B with already made tracks can help aswel. Can be very tricky with already mastered tracks, as you said...

Thanks both. Cheers.....