We're trying to puzzle out the easiest method to distribute our programs among our available servers, that are couple of in number.

Presuming sufficient RAM and CPU energy, can there be any reason we should not run mod_php and mod_rails (Passenger) inside the same Apache server? Would they clash or conflict in some way?

There's little risk with running mod_php and mod_rails on a single Apache server.

Actually, Passenger will not affect Apache whether it goes lower so it's really the ideal choice to make use of with Apache and mod_php. In the Passenger Architectural Overview.

It ought to be noted the Ruby on Rails application doesn't run within the same address space as Apache. This distinguishes Passenger using their company application-server-inside-web-server software for example mod_php, mod_perl and mod_ruby. When the Rails application crashes or leak memory, it'll have no impact on Apache. Actually, stability is just one of our greatest goals. Passenger is carefully designed and implemented to ensure that Apache should not crash due to Passenger.