you would not need a resistor in that setup.
Piezo-Tweeters act like a capacitor and the resistance of the tweeter will only be low enough to produce sound after a certain frequency is reached. If at all you could add an additional resistor in series as a high pass, but you would not have to.
I think you could match those speakers without a second tweeter. Both are specced at 94dB, so they should match.
However, I would not use a piezo as a tweeter, they usually produce high distortion. I would go with something like this: http://www.public-adress.fr/pr-2662/...ts-4-ohms.html
But you would need a crossover for that tweeter and in addition its just my personal oppinion
And I guess you can use a non temperature controlled iron, but you will have to be carefull not to heat the solder/parts too long. But there are persons around here who will be able to give a profound answer to that.
If you don´t fail at least 90 percent of the time, you´re not aiming high enough ;-)