beginner help

[Ethan Robert Duni]

okay; i've got a couple questions i hope you guys can clear up for me; 

i'm looking at beginning my diy career by doing some stompbox type stuff,
but there are a few things that i'm sketchy on:

1) what ranges of signal levels should i expect to see coming out of
guitars/synths/whatevers?  i'm going to be buffering the inputs/outputs of
the projects (which is your standard thing to do, right?), and i figure it's
then a good idea to just throw in the extra resistor and pot and make the
gain of the input and output controllable (or do i need to do both of
those?).  except not knowing what sorts of voltages to expect from the
inputs, i'm not sure what ranges of gain i should provide for.  also,
looking at schematics for this sort of stuff, i always see resistors and
caps in front of the input buffers; what exactly is the purpose of these? is
it for protection or what?

2) powering these things with 9V batteries.  one project i'm doing right now
involves using both CMOS inverters (for a clock) and op-amps (for
input/output and a basic S&H; the plan is to have a basic S&H with a
footpedal to control the sampling frequency, so you can get all sorts of
weird folover and transposition effects).  so i need to drop the 9V down to
5 for the CMOS, right? should i do this by referencing the 9V as +5V and
-4V?  or would it be better to preserve symmetry in the supply voltages for
the op-amps and try to run the inverters on +4.5V?  this seems to be what's
going on in a lot of schematics i see (also, there are usually resistors and
caps between each terminal and ground; is this just to prevent shorting or
is there more to this?)

that's all i can think of for now..

Thanks
Ethan