Regarding both of the above questions...the grayscale clock is 16MHz from the CKOUT pin on an ATTiny dedicated to each row. This project is actually still in limbo, about half the LEDs soldered and the controller boards need to be completed. I didn't get it done in time for Maker Faire. :(

The power supply issue is solved quite easily with an extra computer power supply I had on hand. Those can put out 30 to 40 amps DC at 3.3 and 5V. Even a nice separate 5V rail for my logic boards.

I'm using p-channel MOSFETs to switch the LED anodes, and connecting the TLC5940 sink outputs to the common cathode. Technically, I could have done it with common-anode LEDs. That would require connecting three TLC5940's to six rows of 16 LEDs, with six red cathodes connected to one channel of one TLC5940, six green connected to a channel on another TLC5940, etc. Then I'd have to use a high-side switch to power the common anodes of each row in turn.