Vic in answer to the questions form above:
1) The little blue one is a 12" cleaner closer to 11.5 but called a 12" I would think it might work with a box store pw but you would have to see and to match the nozzles to the size and out put of the machine. If it took a #4 nozzle you would need 2 #2 size nozzles since there are 2 spraying heads in there. I would need to know the psi of the pressure washer and the out put of the gpm to figure the nozzle size.
2) No but you can tune them some what with a nozzle
3) Oversize is using a nozzle or a tip with a bigger hole in it than what is needed to make the machine run at its top pressure performance. A bigger hole in the nozzle we lose pressure by letting more water pass freely through this is the idea of DS making a soft-wash system with a pressure washer.
4) A 25 degree pattern is the spread of the spray that comes out of the nozzle forming a fan shape spray, 25 = 25 degrees wide, 0 15 25 40 degrees are the most popular sizes the smaller the fan the tighter the pattern and the more the water stream is concentrated thus more cleaning power. 25 degrees wide yields a softer cleaning a wider surface area, more surface spread equals a gentler covered area but still offers enough push to turn the bar.
5) I just go which ever way is the best run at the time across or with does not really matter when your cleaning in a circle the water fan will cross the board some ware in the circle with a wand go with the grain if possible though to reduce stripping and crosscutting the grain.
Good luck with your projects. I have a bit more info in a blog that is written for residential customers to help them under stand a bit on how a pressure washer works. I get a lot of question and we have a lot of splash and dash artist in the area. So to help them arm them-self with some basic knowledge and to keep from getting ripped off we started the blog and add to it from time to time. It's simple but does explain some of the nozzles and what they do and how come they work the way they do,
http://cleanedbypete.blogspot.com/ remember this is for the customer so it is a simple and has brief description figure I might get a bit more technical after all the basic info is covered.