Schwarzenegger trained strenuously hard, aged 20, before he could 'at least' be classified as mesomorph. It's hard to determine exactly what body type he was prior to his intense training regime - he could very well have been a classic Mesendomorph (in between meso, and endo) that trained hard to look temporarily like a mesomorph. This is also the case for a lot of 'would be' mesomorphs that only trained extremely hard, and possibly took steroids to achieve that build... such as other bodybuilders like Frank Zane, or wrestlers like 'The Rock'. So yes, I agree, it's not particularly that easy to define a type without a prior knowledge of what they were 'naturally'. A lot of people can put the effort to look like a classic mesomorph, but I'm talking about the pure natural mesomorphs - the 'true' mesomorphs that never required any effort to be that build. True mesomorphs can be of any height, so height is not important for them, however bone size apparently is, because mesomorphs are larger than most meso-ecto's/ecto-meso's/ecto's. A lot of meso-ecto's may be easily mistaken for true mesomorphs, too.
... I think the mandate for true mesomorphs is that they're action-oriented 'fighter' types with the most sophisticated 'muscle tone' out of all the body types - they gain muscle easily because the muscle is 'ready' to pump up at will if you'd say. When you go to the gym for the first time, for instance, you'll know what I'm talking about. A lot of people need at least two weeks (standard) in a 'warm up' phase to adjust the muscles. True mesomorphs tend to get results more-or-less in the first week, and can progress onto the next rep the following week. Someone's career may also determine what build (s)he is, and I doubt true mesomorphs pay that much attention to money.