The Nikon mount was probably the most popular third party mount except for the M42 Pentax.
85/1.8 or 85/2
Yes, from the 70s onward, 3rd-party makers made lenses for Nikon-AI/Canon-FD/Minolta-MD/Pentax-K/Olympus-OM, and less so for Yashica and Fujica. You can easily get lenses for all of those major makers.
Regarding the OP's question: It depends on what kinds of indoor portraits you're taking. For waist-up shots of groups of people, you might discover that your back is against the wall because you need a 35mm lens (assuming 35mm film). For individual portraits, 50mm and higher are suitable depending on how close-in you want the shots. One rule is to avoid getting closer than about 1.3m (4 feet) to avoid the big-nose problem mentioned earlier. For shooting a variety, I suggest using a good wide-to-tele zoom, shot wide open.
I've been testing lenses recently in my home-made autocollimator, and have entered the results in a spreadsheet. Some lenses that perform well wide-open are:
Kiron 28-105/3.2-4.5
Olympus Zuiko 35-105/3.5-4.5
Sigma Pi 35-200/4-5.6 (this super-zoom is a surprise, but two samples tested well)
Tamron 35-135/3.5-4.5 (borderline recommendation for wide-open use)
Tamron 35-80/2.8-3.8
Tokina 35-135/3.5-4.5 (my notes say that corners are a bit soft wide-open)
Tokina 50-200/3.5-4.5
I'm sure there are a number of other fine zooms out there, but I haven't tested them because I don't have them.
Mark