A few issues were raised in this thread.
As far as mesomorph is concerned, it seems an obvious case for reclassification to rare. The word is part of a classification of body types proposed in 1940 by a psychologist named William Herbert Sheldon. The two other principal categories in this system, endomorph and ectomorph, are treated as rare in Chi.
I feel these terms had a brief post-war period of being in fairly wide use, partly relating to Sheldon's theory that personality was strongly correlated with these body types. The OED quotes W.H. Auden in 1952: "Behold the manly mesomorph Showing his splendid biceps off." However these terms are seldom used in recent years. Mesomorph is used very much less frequently than sophomore in publications from Britain and Australia, etc, let alone the US. Thus some of the forumites writing in this topic that they'd never encountered the word.
I'll make mesomorph rare, and remove it as a seed word for 7-by-many puzzles. Interestingly, it never was a seed word for 9-letter puzzles.
Of course, the same letters are used in sophomore, so if I agreed to make that word common, the letters could be reinstated as the seed for a 7-by-many. I'll rule on the sophomore question shortly.