1st) the manifold on the 16v Citroen/Peugeot are the same, but underneath the system hangs diffenetly as the exhaust is hidden on the 106 and aftermarket exhaust hang below the bumper where as the saxo comes through.
This is a picture of a saxo with a PugSport Group N/A system on, note how it hangs to one side and is too low for the saxo back bumper
A decent custom exhaust shop will be able to adjust the hangers on the back box and weld you up some new hangers on the car as to allow the 106 exhaust to hang correctly.
As for trimming a back bumper carefully mark the area needed to be cut and then carefully cut using a small saw. make sure to mask the the area with masking tape as this will protect the paint from chipin too badly.
Remember -
Measure Twice, Cut Once - you only get one chance to get it rite, make it too big and your be stuck with it make the hole too small at first then either trim or sand/smooth to the correct size. Take your time and dont rush it and your be fine