Desktop and Plus compatibility problems and what to do instead

I have been told (per CSR's) on the desktop version that I shouldn't have a page longer than 2000 pixels or the page can be "buggy', with bad spacing. I have also been told after getting to about 2000 pages that there is a limit that the desktop doesn't work at. I have also been told that the Plus version is incompatible (in some ways) with the desktop version - after I have a lot of screwed up pages that I have to somehow repair. It appears that I should only work on one version on any one site. Can I work with no problems on two computer with the desktop versions on two different sites and be sure that all published pages will show up later if I use the desktop version on another computer?
I am confused and befuddled without knowing what is so, what are the limits I should know, etc., so is there any manual or place I can go to get the rules without constant searching among many questions???