Are you seeking to obtain closely cropped, head and shoulder type portraits, or is your goal more like environmental portraiture, or is it something in between?
The rule of thumb that suggests moderate telephotos for portraits is based more on the head and shoulder type of portrait.
Your goal is to achieve flattering perspective, which itself is really a result of appropriate camera to subject distance. Decide on that distance first, and then choose a focal length that fills the frame with your subject.
It isn't unusual to run into space constraints that force a shorter lens than the ideal - sometimes the studio isn't big enough!
By the way, I'll fix the typo in the title, and add a reference to lenses.