You may be somewhat misinterpreting what you've read. Most (recent) wifi radios are good at rejecting adjacent channel interference, so your concern shouldn't be with radios on different channels. An AP on channel 1 will not cause much interference with another on ch 6, for example. The same goes for clients.

A big problem is co-channel interference. That is nearby radios on the same channel. This will significantly reduce your throughput.

You may be somewhat misinterpreting what you've read. Most (recent) wifi radios are good at rejecting adjacent channel interference, so your concern shouldn't be with radios on different channels. An AP on channel 1 will not cause much interference with another on ch 6, for example. The same goes for clients.

Edit:

When two workstations are associated with the same AP, the AP coordinates their transmissions using CSMA/CA. So they don't interfere so much as they compete for available time slots. The more clients you have, the more competition there is for airtime and throughput decreases.

Co-channel interference can occur when there are two nearby APs on the same channel. In this case, the APs are not in sync, so clients associated with AP1 are not in sync with clients associated to AP2. The clients will interfere with each other and the other AP, causing deferred transmissions, retransmissions, etc, all taking a big bite out of your throughput.

Using nonstandard frequencies (like channel 7) is just wrong and will cause everyone problems.

Also, +1 to everything @RickyBeam said.