New Rocket Mod Development

[R_Skoklater]

Hello everybody, so I have been playing KSP for quite a while now and I decided I want to make my own rocket mod,but unlike others I wanted to make it correctly and realistically (most of the parts).

I am no mechanical or aerospace engineer or rocket scientist either so I wanted to ask the engineers and the scientists who play KSP if the way I design and modelled some parts were not realistic.

So the project future rocket will consist of a 5 segment SRB lower stage with a top mounted Cryogenic Upper Stage and a payload fairing just like the Ares I cargo configuration.
However the upper stage will be a hybrid between the single engine Centaur and Atlas V core stage and  Ares I's J2X (skiff) powered upper stage.
The issue I am having right now is figuring out how to make things appear realistically.
Where are the upper stage flight computers located on common orbital rockets and what do rocket upper stage flight computers look like ? (I tried google but I could only find the Saturn V's very very outdated analog computer inside a ring like housing).
Should I allocate the flight computer parts inside the forward skirt of the upper stage? Or, inside the fairings just below the Payload Attach Fitting and why ?
Do rocket upper stages have feed lines and do they also house and connect wires and other equipment or do they just act as feed lines for oxidizer/fuel ?
In KSP we have decouplers to separate different stages. Do interstages act as decouplers/separation system ? I mean do they separate the stages like the way decouplers in KSP do or do they just serve as Structural Support and housing for the upper stage ?  Also, are interstages made of composite or aluminium ?
Why are Payload Attach Fittings shaped like conical shells and are Payload Fairings attached to the PAF or is there another mechanical deployment mechanism they are attached to ?

 

These are overviews of the cryogenic upperstage project I am working on. It currently has the Payload Fairing and the payload attach fitting and the tank is a 5.6m tall tank. The current configuration has the flight computer sitting just below the PAF and the flight computer is slightly raised by a spacer so it doesn't clip with the payload fairings.

Next step of the project will be adding an RCS module integrated with monoprop tanks at the aft section of the upper stage tank for attitude control.
I currently am UV mapping the parts that are ready for texturing but I am still learning how to texture.