No that works just fine. You can open one project in 10.5, another in 10.0 and a third in 8.5 etc. simultaneously.
This is independent of whether you did a full install or update.
Cubase never uses shared components between different versions. Even when you update you still end up with an entirely separate installation for each version. When you update it copies common components from the old installation to the new one whereas when you do a full install it downloads those components to install them. I almost always use the updates because it is so much faster to skip the huge download for stuff I’ve already downloaded once. So far (knock on wood) it has never been an issue.