I don't think ignoring individual differentiation is entirely well thought out.
This flies against everything that astrology teaches us..
I do believe however that the three body types are insufficient to explain all human constitutions and they are a simplification of elemental body types, or which there are 5, at least in Chinese classical theory.
An 'ecto mesomorph' in elemental theory would likely be a mix of wood and fire, strong broad shoulders, wiry lean muscles, with a tendency to frustration and burning themself out quickly.
It will do a person of this phenotype well to do structured, medium intensity workouts coupled with foods which build blood and move qi..
Green vegetables and beef should be the core of the after workout fuel and one should pay attention to burning out or becoming impatient for results as these will likely come over a longer period of time, rather than quickly.
Overtraining is a real risk to the fire element and the wood tends to push this even further.. Keeping a moderate temperament and staying flexible towards your goals is important.
You may also find some benefit in working out with friends to help motivate in a social setting (whereas others prefer working out alone you need some social interaction).
Keep your workouts interesting and work your joints and tendons before you hit the main muscle mass hard.
This is of course assuming you have correctly defined yourself as 'ecto mesomorph'.