Cubase 5 set up for digital piano questions/ help please

Hello all,
I’m quite new to Cubase 5 and would like some guidance with setting up a digital piano, to use as another instrument and also as a midi controller.

So far I have managed to get cubase to "see " the piano. I have midi signal ( via usb) and can record. I can also get GM map and get the standard drum voicing coming back through the keyboard and record them.

But here are my first problems :

How do I control the GM map drum volume independent of the piano? The issue is when I play the drum loop I’ve created back through the piano ( acting like an audio interface) it is louder than my actual piano volume and I can’t seem to change it. Automation and mixer volume seem to have no effect. What am I doing wrong?

Secondly , how can I get different I instrument voices from cubase to sound on the piano, like GM map?

Many thanks