Mix and match rack mount servers

Apart from the height requirements there are three important points to consider.

1. Round or square holes in the rails.
2. Depth of the rack.
3. Whether the mounting rails are compatible with your server mounting hardware, such as slides.

Although I've intermixed devices and racks with round and square holes it's much easier if you have the correct one. Square is by far the most common.

Rack mount devices can vary considerably in their depth. I've had to take the back door of racks before because a new server is just a tiny bit too long. Work out the greatest depth your devices require and then add a bit more to that to be safe.

With mounting hardware you will sooner or later come across stuff that's supposedly designed for "standard" racks (there's actually no such animal) but you won't be able to mount it without modification. There's precious little you can do about that other than be prepared for it. I've been fortunate enough to always have access to metal working equipment when having to deal with this little issue, resorting to cutting and welding on more than one occasion.