There aqre a few books that start a library...

The Higlye series are a good place to start, as well as the ones published by Model Airplane News, and the anthologies from RC Modeler.

And yes, every plane is somewhat unique.

Pica kits are known as 'builder kits' where you're expected to have a pretty good idea of what goes where. the instructions are somewhat of a formality. My Pica Spit is teh same way, incuding a few steps out of order.

As for retracts, it's not uncommon to add a few 'structural members' as and where needed. Try to install the air tank, the valve, and the mini servo in the wing center section. Or, if in the fuse, it'll be one more connction to make every time you arrive at teh feild, and will need to pumpo it up every time too. And yeah, the diagram is really all you need. it's up to you to figure out how the lines should run... To run them the best way through ribs, around spars, etc...

But, ask your question! that's what we're here for!!