Hey g.i joey, I thought I'd just state that I'm very happy to read that you can take 0.25 mg daily of Finasteride without any trouble. Has that dose of the medication been helping you? Have you noticed any differences with your hair and scalp?
About the theory in your OP, to be honest in my opinion, I don't think whatever somatotype a man is would make a difference on how prone they are to gynecomastia. For example, look at FredTheBelgian, he's a 6'3" ectomorph and is extremely prone to gynecomastia. You're an endomorph and prone to gynecomastia. Like I previously stated, I don't think whatever somatotype a man is would make a difference on how prone they are to gynecomastia. I'm not knowledgeable about the condition, though I believe genetics and how sensitive they are to anti-androgens and/or how sensitive they are to lowered androgen levels play a role.
I myself take Dutasteride as you're probably aware of, and I've had no trouble with the medication. I'll roughly estimate that I'm approximately 70% ectomorph and 30% mesomorph, and I've been completely fine taking the dual 5-alpha-reductase inhibitor. As I'll state again for the third time, I don't think somatotypes and gynecomastia have any correlation with each other.