Strangely, I don't think the value really matters since the pot will be used for blending.
What power doesn't come from one piezo will come from the other, so in essence it should balance itself.
With the preamp, you don't need separate volume/treble... the preamp does all that for you...
Given the high impedance of the piezo, I'd guess a larger value for the blend pot might be better, but Francois is probably right... Truthfully, Piezo Rod is great, I don't see any advantage to adding a disc, unless you like handling sounds and feedback...
just make sure the blend pot is a linear pot and NOT an audio taper (log) pot. otherwise it will be difficult to get a blend you like..
The round piezo and rod piezo have different outputs(round piezo is louder and more likely to pick up all handling noise). If your using a preamp from Gitty, I believe these have only one input. This would require the parallel hookup making one louder than the other.
So a blend pot between the round piezo and preamp input would work. You could use a standard volume pot and wire it: piezo signal to lug 1 of pot, lug 2 of pot to preamp and lug 3 of pot to ground. That will work as a blend.
As for the value of the pot: I would try a 500k first.
I got that you weren't asking about the value of the pot to be used at that point, I listed it because of the impedance coming from that piezo being higher, the 500k would help to minimize that issue.
The pots for the preamp system are hard to determine without seeing a schematic. Stewmac and other places might recommend 25k pots for Battery operated preamp systems, but they are usually for EMG style pickups with built in preamps. The preamp your talking about could use a 1k, 10k or 25k pot or a mixture of values. So a schematic would be helpful.