I have 2 x Maxi Cosi Priori SPS seats (because of twins) in my 5-door 147, and I find them to be very very good, compared to some other seats. I have no issues about buckles. Lots of other baby seats can not be properly tightened to a car seat, and I find maxi cosi system to be very good in this field, especially since it is without IsoFix. Priori XP is even better compared to Priori SPS, since it is basically a same seat, but with an additional handle for belt tensioning.
Also, I can't see why fitting to a 166 would be a problem...I have tried to move the seats in some other cars (33, Renault Clio, Scenic) and found no problems.