I'll give some of these a stab
- Since bolters and bolt pistols use the same 'caliber' of bolt as ammunition, is it fair to assume then that the magazines are interchangeable?
There are different "marks" of bolters and bolt pistols. I would imagine that the magazine from an astartes bolter would not fit into the handle of Gaunt's compact bolt pistol.
In general bolters are only carried by astartes because of weight and recoil. Although I'm not certain I'd say that apart from maybe the ammunition a "human" bolt pistol is nearly completely different to an astartes bolter and maybe even an astartes bolt pistol.
So to sum up Astartes gear probably not interchangeable with human but probably interchangeable with other astartes gear. And the same goes for guard issue gear.
- How much ammunition does a heavy bolter's backpack contain? Given the firing rate, how quickly would a Devastator Marine deplete his load?
No idea, the size of bolt rounds and space marine dimensions vary from source to source so I couldn't even hazard a guess by "mathing" out the volume of the ammo bandolier on a devastators back. I would say it would be completely unrealistic though.
-Where is the power supply for the SM's armor: underneath the backpack?
The power supply IS the back pack its called a "power plant" or something....
- I seem to recall from Rogue Trader that a Space Marine's power supply also housed stabilizing jets. How effective are they? Can they be used to jump to higher, or survive a fall from a greater height?
I remember that one too, vaguely. I think it was a throwaway comment. Space marine back packs are not jump packs, it wouldn't be the first time a writer got excited.
- What is the real deal with teleporters? The rulebook states that only Terminators can use teleporters, but many of the BL books (Fallen Angels and Warriors of Ultramar to name a few) describe regular Tactical Marines getting teleported. /confuzled
In game terms I think that rule is there to prevent mass teleportion and make SM's overpowered. In fluff terms any SM can do it and I have a nagging feeling that humans can too but can't remember whether I read that or dreamt it up.
Hope this helps.