All the PC racks I've dealt with have had the large punched holes that you insert rails into. However, I have a seriously nice rack that used to have audio gear mounted in it - which has tapped screw holes all the way down each side.
I'm really only familar with the typical "2 post, round hole" very basic style of racks. The ones I've worked with have threaded holes running up and down them the whole way.
I know that when you buy from Dell, which I do exclusively, you have to get a rail kit that is fitted to match the type of rack you are using.
The rails are lined up with the rack and you bolt them into place. You then drop the server into the rails and bolt the server to the rail system.
Assuming it's a standard with with standard spacing, which you say it is, I don't see any reason why you wouldn't be able to mount networking equipment into it.
You have to keep this in mind though, some servers, like a poweredge 2850 are made to fit into a four post type set up. They mount flush to the front of the rack and have a bit of the server hanging out the back side of the second rack. This makes for a very secure and stable mount.
If you have a two post rack, you have to juryrig it a bit and do a balanced mount where you sort of "center" the weight of the server in the rack. It sticks out both the front and the back of the rack. It adds balance, but is far from being stable if the rack isn't bolted down and counter sunk to a wall or ceiling.
The other thing you have to keep in mind, is that with half racks, like your audio equipment rack, is that they are designed to hang on a wall. When you do this, you have no room in the back. They basically stick out a couple feet from the wall and that's it. You have to get a server that is explicitly designed for rack purposes so that it will fit in that space.
A switch, UPS, or patch panel will fit in that spot. So will most of your audio equipment. A large server that is rack-compatible will not. It needs room to stick out the back side. Some of the smaller chassis rack servers are designed to work in these setups.