Ahh, well that's a different question entirely
Yes, if you install Windows 8 PC as a "dual boot" then it will change the boot record for whatever you have installed. This is an important consideration if you want to try out Windows 8 and test it for Microsoft, because this W8 PC will expire sometime before, (or shortly after, I am not sure), the retail version of W8 is released.
The expired version won't work after that date but it won't affect the running of your other OS. The dual boot menu you see when you turn the computer on will still allow access to your other OS. In fact you could simply erase the files on that disk or partition where you installed W8. The problem is, it may not be easy to remove the dual boot menu without a complete reinstall of your own OS.
Caveat. I do not know how installing W8 CP will affect the running of Linux. I am talking dual boot wth other Windows OSes here.
I'm testing W8 but I have done it this way. I used another hard drive and reconfigured the BIOS to disable my Win 7 hard drive from booting, so that only the hdd I intended to install W8 was seen. I installed W8 on that and it works fine. Each time I want to use the 'other' OS, I access the BIOS, disable the hdd I do not want to boot, enable the other hdd, and then exit and save. Then the OS I want will boot.
As to your 2nd question, W8 is quite different initially, but it seems stable. It is much like Win 7 underneath the Metro GUI you see when W8 boots up, but the other features of Win 7 are still there, if slightly rearranged.
W8 is designed primarily for Android phones and tablets and touch screen technology, and that is what the Metro interface is for. But the Desktop/Laptop PC OS is still there.
W8 CP is still a Beta so I would expect some instability. It is not the final version, so don't download and install it if that is what you think it is.