anyone think its possible to use the gyro from a piccoboard in a trex?

Without a large electronics modification effort? I think not.
The output of the piccoboard is power for a tail motor.
You want something to drive a tail servo.

Also, not being a true head hold gyro, you would not be as happy as if you used a Futaba gy401 or other capable head hold gyro.

I take that back. It IS doable.
http://pgoelz.com/piccolo_separate_tail_esc.html

Now, is it desireable? That part I still stand by from my post above. A true head hold gyro will give you a much better behaved heli.

ok then how about this. what exactly is the diffrence between a normal tail gyro and a one with a heading hold? dont normal gyros hold there heading until they recieve input to change it?