They did alternate the seating during the first 3 weeks, but never had two males or two females sit together. One or two shows, they had F, M, F on the top tier and M, F, M on the bottom tier. So, Richard Dawson did sit in what later became Brett's spot for one show, and also CNR's spot for another show. My theory is, Mark Goodson or someone realized that Richard, being arguably the funniest of the panel, SHOULD be on the bottom tier, so that any funny business he did could be saved for later, and also the rest of the panel didn't have to try to be funnier than him.
Brett also didn't originally sit next to Charles when she first started. She was on the lower left. However, an innocent remark about Charles "not wearing any socks" got the producers thinking that PERHAPS they should try seating them together, which worked beautifully.
Only twice later in the run did they switch seating. Sometime in 74, the panel all switched seats during the intro, as a joke on Gene. Gene didn't like the new arrangement and make everyone switch back.
A year later, Richard Dawson suggested to Gene that everyone switch places. Most of them did, but for seme reason Brett stayed where she was.