...Builder B did state that I didn't want a larger filter than the 150 ft2 because it wouldn't work...
That one statement gives me real concern about their competence, as a rule of thumb it is impossible to have too large of filter, this is particularly true with DE and Cartridge filters. In the case of sand filters you must make sure that you have a large enough pump to backwash the sand filter, but that is rarely a problem, and things would have to be taken to absurd proportions for it to be an issue.
I think I should leave it to people with experience in your local climate on the below ground level, decking and paver topics as such things do vary by region. For example the use of flex PVC seems to work well in your region and is helpful where the ground freezes and heaves, vs where I live in the south where hard PVC works much better as flex PVC rots and is prone to being eaten by termites.
As to plumbing pad layout, many new pool owners initially like the idea of an ultacompact equipment pad that is tucked into some out of the way corner, however when it comes time to replace things, they find it a nightmare, unfortunately many pool builders seem to think those ultracompact pads are great too, most likely because they are not around 3-5 years later when that pump motor goes out. Now if you have a VERY small yard that extra 10-15 sq ft of space may make a difference, but if you don't make sure things are spread out, that there is enough room in the plumbing to cut out sections, add new equipment, and that is it is placed far enough from walls that if the new pump, heater, SWG, etc. is an inch or two longer that it will still fit. I am also a proponent of using lots of PVC removable unions, although there is an opposing school of thought that says more fittings are more potential leaks, either way spread things out. You want it to look like the engine bay of a 1950's car where you could climb in and work on stuff, not a modern car where you have to take the wheels and fender off to change an alternator.
Ike
