I wanted to standardize on a single type of rack, so I looked for a way to mix servers from multiple vendors in a single rack type. This means the Compaq rack can have sections of Compaq servers and sections of Sun Netras within the same rack. I have not tried mixing other brands of equipment, but I do think the Compaq kit would enable this, as long as your equipment wasn't deeper than the Compaq rack. As for backups, nearly every one of these platforms has at least wide SCSI for tape devices and 100Mbs for over-network backups to some remote device. Because they all can take two Ethernet connections, you could use the second interface for a separate backup network.
Network wiring is no big deal.
I just make custom CAT5 patch cords that run from the servers to the network equipment. Each UPS unit is 5U each, so this combination eats up your U's pretty quickly. Make sure to put the UPS on the bottom, because they are quite heavy. All this adds up to a very heavy cabinet. I use a rotary hammer drill and vacuum up the dust while drilling.