If you actually want the same room effect on all (or most of) the instruments, that's not how I would go about it. You need as many reverb VSTs as you have virtual instruments that way.
A better way to do this is to
- Add an FX channel
- Add A reverb on that channel and set it to 100% wet
- Select all the tracks you want to have reverb on at the same time
- On one of the selected channels, click on the + to the right of the 'Sends'
- Select the FX channel with the reverb
This way, all selected channels should now send a copy of their their signal to the reverb. You can control the individual channel's reverb by adjusting the sends level, and the overall reverb loudness with he fader for the FX channel (or an additional Mixtool, whatever you refer).