There are multiple subspecies of A. dichotoma, and the differences between these subspecies are covered in a book 세계의 장수풍뎅이 대도감 published in Korea. Wish I could get a copy of this book and tell you the descriptions in it, but the book is too expensive for me to afford
The ones in taiwan are subspecies tsunobosonus by the way. They tend to have more slender thoracic horn compare to other subspecies so that's one way to tell them apart from others, but with the increasing number of trades I'm beginning to wonder if the species available in trade are actually pure.
I can post a picture that may be of some help or it will just make things more confusing.
Locations shown from left to right; Allomyrina dichotomus tsunobosonus from Taiwan, I believe Allomyrina dichotomusseptentrionalis? from China (Someone correct me if I am wrong), and Allomyrina dichotomusseptentrionalis from Japan.
I don't have a comparable male from Japan to show since mine is so small so just keep in mind that the small size is not a key characteristic.
I thought the one on the far left was Japanese and the one in the middle Taiwanese? I've heard that breeding different subspecies leads to setrile offspring, is this right?
Subspecies should have no problems breeding and producing fertile offspring if mixed - probably best not to do this in culture as you then have "hybrids" of the two subspecies.