As far as R-2 goes, he just needs to slow down a little and shorten his stroke. Almost all of my R-2 partners are bigger or smaller and compensation is always needed on the part of the bigger/stronger person to keep from spinning circles and or making the smaller person work themselves to death.
Im a little confused as to what you are actually asking? Maybe its just the typos but....
If you are asking if learning to row on different size boats matters, it does and it doesnt.
It doesn't in that when you are first learning you are figuring out how to read water, what the oars do when you push,pull,etc., what the water does to the boat compared to what you read, timing. blah blah.
It does in that if the boat fits it will be easier and more comfortable to do the above. I grew up rowing boats that were set up for people bigger than me. When I setup my first boat for me it was more comfortable to do all the things I had learned.
If this isn't what you were asking, I missed the question completely.