Is there a way to disable/hide the new Photos App in Yosemite?
You can try running this command from Terminal:
defaults -currentHost write com.apple.ImageCapture disableHotPlug -bool YES

To re-enable it again, type:
defaults -currentHost write com.apple.ImageCapture disableHotPlug -bool NO

Just a warning that I don't have access to a Yosemite machine to verify this.