For many authors, the current practice is to set a cutoff point and simply not support versions older than that. Alas for you, SketchUp 8 is largely considered to be obsolete. Too much hassle testing and then supporting code that spans old versions (8 is, after all, 4 versions behind!).
There are, of course, API fixes and tweaks from version to version. The release notes for each version describe these. Possibly the largest change though is that SketchUp converted from Ruby 1.8 to Ruby 2.0 in SU 2014. I won’t try to list everything you might have to deal with. If you search the web you can find discussions of converting old 1.8 scripts to 2.0 and of making 2.0 scripts backward compatible. One very important change was to full support of UNICODE characters. This required significant rework of how strings and characters behave, which broke many older plugins.
Regarding your other questions, you can install and run multiple versions of SketchUp on the same machine without first uninstalling anything. The only problem I know of is that on Windows the most recently run version may reset the “open with” to itself, potentially causing confusion about which versions can open your model files.