I see one problem with this: OpenXCom doesn't propose alien interrogations if they won't unlock any research topic.
Here's my work around, it happens automatically. No actual interroation/research is done. All you need is an Alien containment, a live alien, and all other possible research topics from the subject must be exhausted.
So if you captured an etheral commander, already had the location of Cydonia, but had never caught a live etheral or an alive sectoid leader/commander, you would not get a base location because you don't have the medical entry for Etherals nor the psi research.
If the aliens have an active base, the captured alien is automatically interogaited and killed. This happens automatically until all current bases are discovered.
One possible limit is that aliens can only give info on thier own race. So no Muton will ever tell you about a Floater base.
Like some suggested, you could have other ranks with the info.
Here's one possible chart:
Terrorist 1%*
Soldiers 2%*
Medics 5%*
Engineer 10%
Navigator 30%
Leader 50%
Commander 100%
*All Medical entries must have been discovered
**All ships must be discovered
