Distro -> the one you can work the best. As long as you know how to modify a distro one is as good as the other
Old does not mean bad as long as that fork is maintained. Just look if the desired Version is still maintained.
And wouldn´t it be common sense to look if the mentioned patches are available for newer kernels too and replace the written version numbers with the current?
being an admin is not copy and paste how to´s. It is also understanding what it is written and beeing able to replace things if needed for your system.
there didn't change much since 2.6.32 w.r.t. the relevant kernel config settings. so the howto still can be used with newer kernel versions.
regarding the distro: many people have reported problems with centos that were gone away when moving to debian. centos uses very old (but very stable) versions of glibc etc., which can be a little bit of a disadvantage for game servers. I would recommend debian, but that does not mean you cannot get your desired result with centos.
Debian is already on the server. So the Kernel has to be downgraded.
I think it will be 126.96.36.199-rt29
If you would have read the wiki carefully you would have noticed that RT is not the best for HL1. Also it is better to use the latest Kernel and its RT capabilities if you want to go for RT.