So a player comes to you with a proposal for a ghoran PC, and you're going to hand him 20 lvls of Commoner NPC as a bonus?
Nope. He'll be a level 1 PC, like every other PC. He'll have the usual Ghoran racial benefits, which include potentially being able to benefit from the Knowledge skills of its past lives.
Take the mechanics out of the equation for a second. Look at the real world. We have, in the age of information, access to centuries of knowledge that we have accumulated, recovered, rediscovered, and/or created reasonable facsimiles of things that our ancestors have done since even before the dawn of writing. We know or can learn about the ways and techniques of agriculture, warfare, of generations upon generations of humans on this planet. You could go to a library- or even just Google- more data on these things than people could even 30, 40, 50, or 100 years ago.
Does that suddenly equate to you being some kind of superhuman that can conquer the planet? I think not.
Not to mention if you read what ghoran's are, a significant portion of that experience is combat practice - being hunted, and eaten when they fail to evade the hunt...
Being hunted and avoiding being killed does not necessarily equate to "combat practice." Ask any fox, quail, duck, deer, pheasant, turkey, or squirrel.