Is it possible to mine multiple pools with certain GPU's?
Lets say mine ZCL on GPU 0-3 and mine Zcash on GPU 3-6
yes. you can use the --dev to specify which devices to use.
so just run zm twice with different pool configuration. in the first use parameter "--dev 0 1 2" in the second "--dev 3 4 5".
for more details run "zm --help".