I installed COSMIC Atomic fedora on one of my laptops and am very much enjoying it. Except for not being able to play videos on most of the websites I want to watch videos on. Keep getting source errors. Is there any way to install rpm fusion packages in an atomic version, or maybe a spin of the spin that has certain nonfree packages like codecs added already?

The dnf utility is replaced by rpm-ostree for atomic Fedora. It's not recommended to install user applications with it (you can, but it can mess up future updates), but from an end user perspective all the commands and options are identical. So just add the non-free repositories as normal and install whatever codecs you need.