Installing early 2000's game on High Sierra

Hi. I've recently found an old game (I think 2002-04?) that I'd like to install on our new iMac, running High Sierra. From what I've read so far, I understand that I need Classic Environment to install a game this old and I've seen suggestions to use OS 9.

A few questions...

• Does the Mac stay in OS 9 or is it something you can switch between easily?
• Will using OS 9 in one user, change all users on this iMac to OS 9?
• Could switching to OS 9 cause any problems for the Mac?
• AND, this particular disk was used on a PC back in the day, but says it can be used on Mac as well. If it was already used on a PC is it a lost cause? Can it still be used on Mac?

Thanks in advanced!