You also don't want to cross 5.25 inchers that low; the frame/surround width between the cones will come to about two inches, even less with squared 'pin cushion' frames, so you can go to 6kHz easily assuming that the driver 30 degree off-axis response is no more than -6db. Crossing over that high gives you a lot more tweeter choices, both piezo and dynamic.

The downfall with piezos lies with their roll off slope; with side by side woofer/tweeter lines you really want 18dB/octave or better crossovers to minimize output frequency overlap that will cause horizontal plane comb-filtering. That's another good reason for crossing the woofers higher so their acoustic roll off can contribute to the total slope.