.....t

surprised to know there is a eucalyptus that looks comparable

When I look at them side-by-side, the Red Grandis has a redder cast and the Mahogany looks a bit more yellow. By the way, I think "Red Grandis" may be the commercial trade name the wood is marketed as.

so you have six chairs and three different woods
seems like the last wood choice just needs to be close to one of the
other two

Normally, yes, but the chairs aren't all going to the same recipient. Two for me, two for a friend, and two for a friend's brother-in-law. They won't be seen together.