Q: Will more than one PIC in a circuit interfere with the other's operation either through the traces or via EMF?


A: No. Not if you have taken any care at all in PCB layout.



Q: If I were to use a PIC as a master to send a digital output to the pin of a slave, is it advantageous (or imperative) to have both of them running off of the same clock signal? How about a master outputting to 2 slaves?


A: This is not necessary. All can be running off separate clocks.


Q: Is there an electrical limit to how many PIC's can be clocked together, or for that matter co-exist on the same power bus? This assuming the bus is correctly sized for adequate capacity.


A: None.



Q: is a simple crystal oscillator package capable of providing clocking for more than one PIC on a clock bus, or does a more complex clock module of some design need to be introduced?

A: When you say "crystal oscillator package", I'm assuming you mean the "can" oscillator types that have power and gnd pins.
One of those will clock a lot of PICs. If you are talking about a "bare" crystal, then there are some very real limitations. In that case, you can use the CLKOUT of one chip to drive the CLKIN of another. I believe MChip has an app note that deals with this.