Where to begin...
First off, what everyone going into these issues MUST know, the part-perforated and imperforate varieties of the US First Issue Revenues are BEST collected as imperf between pairs or larger multiples. There is no other way to be assured you have a genuine copy instead of a wide-margin perforate example trimmed down to make an "a" or a "b"
I do collect single part-perfs and imperfs, but only at very good prices.
Examples of singles tied to documents are also pretty safe.
These varieties were produced because in the first few months of use, the printer could not keep up with his contracted-for requirements... their perforating equipment was slower and less efficient than their printing presses... they delivered stamps in whatever state they had.
That being said... part-perf and imperf varieties are in some cases more common than the fully perforated varieties.
That is because the "Matching Use" requirement for the documentary titles was dropped after 18 months or so as being totally unworkable in the field.
That means that some titles never or rarely got sent back to press after the requirement to use a Mortgage stamp on Mortgages requirement, (as an example), was dropped.
It also means that part-perfs and imperfs SHOULD show up only on old paper, not experimental paper.
You should also know that in many cases, imperf proofs are less expensive than issued stamps... and are often seen sold as issued stamps... you need to recognize the correct paper also.
I collect and study ALL revenues... not the just the 1st issue. I'm more concerned with filling out all face-different, (8 different types left). I know that some of my single perf varieties are suspect.
You can choose to collect these in any manner you'd like. Just don't pay any premium for an imperf variety as a single no matter how they are listed and priced in Scott. I suppose when you see a copy with frame line or more of the adjacent stamps on BOTH sides... it would be pretty safe.
I use the National Revenue pages which includes seperate sections for all varieties as singles. You can just remove any of these pages you'd prefer not to deal with.
I use all the pages as blank spaces certainly don't disturb me

... and I insert blank quadrilled pages for multiples and documents.
I imagine that
revenuecollector will show up soon to give you a link to his very nice US Revenue reference charts... I'll not steal his thunder.
I don't know if he indeed collects all forms and varieties, but it's obvious he studies them all extensively.