Disclaimer, Im an amateur.
The T-55 uses later gun model with smoke extractor, the SU-100 doesn't have it. Therefore, the gun will not match. I don't know ifSU-100 has different gun mount system than the SU-85 considering the D-10S is significantly heavier.
The commander cupola of the SU-100 is the same as the T-34/85, like the SU-85M in fact. SU-85M is the 85mm version of the SU-100 by the way, and I don't remember any SU-85M from Miniart with it.
Most of SU-100 has a ventilator behind the commander cupola, which look like the one on T-34/85 I believe.
Driver hatch on the SU-85 is also different from the SU-100, but I remember somewhere say that it is depending on factory.
So, in my opinion, it is not really possible to make a SU-100 by combining SU-85 and T-55, useless you don't mind a fictional, plausible SU-100ish.