It might be on something of a tangent to the post-1979 TV/Movie trekverse, but the version of the Klingon and Romulan Empires seen in the Star Fleet Universe are fleshed out in various RPG sourcebooks
... though the book for the Federation proper would be nice to see eventually.
In their setting, the Klingons
are the more centrally-organised society, with varying populations within the capital complex, on major colonies, and in other parts of the Empire - as well as on colony worlds licenced from the Far Stars Duchy of the Klingon-allied Lyran Star Empire
Also, the Empire is shown as having several subject races, of varying status and position within the Empire - from the bookish Dunkars to the radiation-poisoned Cromargs and the radiation-tolerant Vudar.
Generally, subject worlds are allowed to run much of their own affairs, so long as they pay their tributes to the Empire, don't try to rebel, and provide a steady stream of recruits for the Deep Space Fleet.
(However, the DSF restricts most subject race crewmen to serving positions as Marines, technicians, and so forth - and even keep security stations on each Fleet ship in order to watch for potential disloyalty.)
, in contrast, have no formal subject races in that setting - their 'Remans' are identical to Romulans and live on a less harsh world than the Remus seen on screen.
Well, aside from one little mishap involving the USS MacArthur
, that is...
However, there are a small number of what the Federation call 'Suppressed Races' - worlds which are at varying stages of development, but which the Romulans have not formally subjugated outright.
(By the time the Romulans discovered them, there were more immediate priorities for the Empire to trouble itself with.)
Neither place would be as nice to live in as the Federation
, Inter-Stellar Concordium
, or other, more pluralist societies in the universe - but as seen on TV during the Dominion War, the needs of survival meant that the rival powers had to co-operate when an external invader
poised on conquest and subjugation rolls in...