It appears that most of the class files don't exist in the form of a rpm. In fact, some of this stuff doesn't seem to be downloadable as a .cls file (which is the format that lyx expects to see). Somehow (that I haven't yet read up on) you are to convert a tex file that you download from a random ftp site into a cls file and that goes into a directory that lyx can see.
Obviously, I have some more reading and figuring out to do here. A lot of this stuff was apparently never packaged for easy installation on a rpm-based system.
LyX is just a front end GUI editor for LaTeX/TeX. You could try installing some other TeX packaging system - on CentOS7, that would be texlive - and that will give you many of the other files and classes you may need.
Other non-packaged files can be downloaded from CTAN.
I think what you need to get your head around is that these aren't "LyX files", they are "TeX/LaTeX files" that LyX uses.
P.