Well it is Strathfield, they're not much different to Northfield ... very inexperienced workers with lack of commonsense or logical reasoning... They could just be lazy of course.....
In my experience, there is no fail-safe company to go to for installation work. I've seen stuff from WOW, Autobarn, Strathfield and GCCS all with various things that weren't done right, everything from RCAs run alongside power cables, to speaker grilles not screwed on properly, to over rated fuses on power cable that was too small, to improperly crimped terminals, to bad grounds (paint not removed before grounding), to just plain crack-headed wiring.
In the SE Qld area Northfield, Maroochy Car Sound (I think it's now a Strathfield store) and GCCS have a fairly good rep for custom work, I'm probably a little biased but Strathfield @ Slacks Creek's installation jobs are tops
Finally my opinion: 6" speakers usually do fit in 5x7" holes, cutting usually isn't required but if it is it isn't by much and I'd do it anyway. Laziness doesn't really come into it because with the tools they have cutting the holes larger literally takes about two minutes. I'd hazard a guess that the reason you were sold 5" splits was they had a better profit margin and netted the salesman more commission.