A 555 timer can drive a piezo if the load is sufficient.
There is a propagation delay for the piezo to actually make sound.
If a trasistor is needed just use a 1N4007 NPN general. Drive the base with 2 resistors to divide voltage. Since the voltage output of a 555 is realitivly high drop in a inline resistor to drop the signal output of the 555.
Use 2 100kohm to divide the VCC equally into the base, this does not require coupling, but pay attention to divider current if it is to high it can fry the 555, I would work the voltage and resistances to equal .75 volts base to emmiter junction voltage, just bearly enough to activate the transistor, and be under 50uA of current and the input to the trasistor form the 555 to equal about double or 100uA or more. Yes I am getting at a common collector transistor set-up, using it that way will make sure you get enough current at a given voltage to drive the piezo.
On the other hand I still trying to figure out what you are try to do exactly.