"A matter of euphony" means he did it because the name of the new partnership would be Cromarty Ross, which to him, because he was Scottish, sounded very pleasant. "Euphony" means "pleasing to the ear, because of a harmonious combination of words". "Ross and Cromarty" is a beautiful area in western Scotland.
If there are other specific phrases you would like explained from this passage, please ask, and I will do my best to answer. :)