Can you get a virus just for visiting a sketchy website?
Also, some programs aren't available via my package manager (I use Fedora) so I have to add 3rd party repos. Is there a general security guide for linux?
Nowadays it is almost impossible to get a virus just from visiting websites.
As for security recommendations I would recommend never running applications as roo that 100% don't need it, as for 3rd party repos I would always be a by mindful of the apps but generally there isn't too much of a risk, of getting a virus.
Highly unlikely. A site could try to exploit unpatched security holes in your browser, but if your browser is up to date, this is unlikely to succeed. Modern browsers are very complex and large so they have lots of weaknesses, but they also get fixed quickly, a lot of eyes are on their code and they utilize sandboxing techniques as well to isolate things from your system.
Still, it's a good idea to harden your browser further yourself, or run it in an additional sandbox.