Eric,
You are correct in that the "Tei" varant had the 20mm cannon installed. The "KAld" version had the 30mm cannon installed.
Don't know if you have markings already for this picked out, or how far along you may be in the build, but if you must have 1945 Home Defense, then there's an uncommon unit you might be interested in. Starting in November 1944, the 10th Hiko Shidan was ordered to ram incoming B-29s. They flew Ki-61s at the time, and stripped the fuselage guns and armor out of those planes. Within the 10th Hiko Shidan, the 244th Hiko Sentai was put together as a special ramming unit known as "Hagakure-Tai". The unit did operate Ki-61s and did operate in 1945, having some success in ramming B-29s.
From what I can tell, the main differences between most of the different versions of the Ki-61-I were the wing guns. There was a tailwheel landing gear change from the original model to the -Otsu, but aside from that, the only change in the -I models seems to have been the wing guns. That means that your model should work for any of the units that flew a Ki-61-I variant, as opposed to the -II variant, many of which had a different wing, and all of which had a different engine than the -Is had.