Hosofos on Parshas Hachodesh-Rosh Chodesh

Rav Hamburger told me he is not aware of any difference between Parshas Hachodesh when there are 3 Seforim (and we normally have 6 Aliyos in Parshas Hashovua, and the seventh in Parshas Rosh Chodesh) and other weeks.
In any case Lekatchilo one should not add an Aliyo above 7 Aliyos on Shabbos, and if there is a need - one Aliyo can be added.

It would be interesting to see the source and the reason given there.
In Zichron Chanoch which brings the Ashkenaz Minhogim for Kri'as Hatorah, it is clear that there is an option of Acharon on days with three Seforim too, (see for example on page 7 Parshas Miketz, and page 9 Parshas Mishpotim and Terumoh).