How the ki/chi thing works for the Xing characters is that normal humans have only one soul. The homunculi have hundreds or thousands of souls, and that's why their ki is so distinct. In Bradley's case, he's a homunculus with only one soul, which is why Ran Fan thought he was only a human at first. In Al's case, they noticed that Al doesn't have the "flow of ki" that most humans do, because he's only a soul attached to a suit of armor. And for Father...well, he probably has tons more souls than the homunculi do, so that might explain Ling and Mei's strong reaction to him. Or it could be even more than just having a horrifying number of souls, because after all, he's not a normal "human."
Also, in Chapter 40, Fuu says that before the Western Sage came to Xing, their traditional alchemy was only "products of delusion and imagination" and couldn't even be clasified as actual alchemy. "They believed that mercury had the effects of immortality to the point that there's a story of three generations of the imperial family continuing to drink mercury until they died of mercury poisoning." But when the Western Sage came to Xing, their alchemy began to progress. The Rentanjutsu of the present is a combination of the traditional alchemy of Xing and the alchemy that the Western Sage brought over.
The alchemy of Amestris, however, was brought over by a survivor from Xerxes, who would come to be known as the Eastern Sage.
Now let's say that the Eastern Sage was Father. If Father was the one who taught the people of Amestris alchemy, then it wouldn't be difficult for him to decieve them into using an artificial form of alchemy that he can control (turning it on or off). After all, Envy said that they're using it without even really knowing what it is.
And let's say that the Western Sage was Hohenheim (because it seems Father doesn't know about the Xingese alchemy). Hohenheim could have taught the Xingese the "pure" form of alchemy from Xerxes, and with the Xingese emphasis on medicine, it would have developed into something new.
Now what's interesting is that the source of power for Rentanjutsu comes from the ground- a power called "lungmei" that exists all over the earth. Scar's brother noted that it sounds similar to the god Ishvara, who is an earth god. You'll also notice that Mei always draws a circle in the ground or plants her daggers in the ground in order to transmute. So is the earth the "pure" source?
Then what is Renkinjutsu's source? Is it Father? Mei says that there's a flow of power inside humans just like lungmei (she's probably referring to ki), so what if Renkinjutsu's source is the flow inside Father? Father seems super-powered enough to be able to support enough energy for people all around the country to use, and maybe that's the reason he's plugged up to the tubes? Because he needs to recharge? Maybe that's why he seems to be physically aging more than Hohenheim (assuming they're the same age)? But then again, Father also stamped his foot on the ground to cancel out Amestris's alchemy...
And what about Scarbro's alchemy that mixes Renkinjutsu and Rentanjutsu? Could it be that his hybrid alchemy gets its power from the "lungmei" of Rentanjutsu, but its technical aspects reflect that of Renkinjutsu? If that's so, then it would of course resemble the alchemy of Amestris to the point that Ed and co. wouldn't notice any difference, but Mei would notice because she can recognize its source.
[Quoting MonsterEnvy: Renkinjutsu does not heal, and rentanjutsu cannot reattach limbs, but Scar's brother manages to attach his own arm to Scar, which is something that's a combination of both.]
It's true that Rentanjutsu specializes in medicine, so it probably has more capabilities to heal, but Marcoh was also healing people with Renkinjutsu. Of course, he was using the Philosopher's Stone as well, but when Roy also tried using the Philosopher's Stone to heal Havoc, he said that healing alchemy wasn't his specialty- he wanted to use the Stone to make up for that. So Renkinjutsu can be used to heal, just probably not as well as Rentanjutsu without an energy-booster like the Philosopher's Stone. This might be why we didn't see any healer alchemists in Ishbal, and normal doctors were sent in instead.
I do agree that it's probably because of the combination of Renkinjutsu and Rentanjutsu that Scarbro was able to attach his arm to Scar. Mei's line about not being able to grow an arm back with Rentanjutsu might have been foreshadowing for that part of the flashback. Which makes me wonder- if Ed and Al manage to mix the two types of alchemy together, would they be able to reattach Ed's limbs too?
In the recent Ed discussion post at fma_reread, nearly everyone (including myself) thought that Ed wouldn't get his limbs back in the end, but if we're right about that aspect of the hybrid alchemy, that would present a believable way for Ed to get his limbs restored, wouldn't it?
And if Ed and Al learn to use lungmei as the power source for their alchemy, would that be enough to open the Doors without making a sacrifice...? It wouldn't be that easy, I'm sure, and there would probably be consequences for taking so much advantage of that power. If it were so easy, then the alchemists in Xing might have already been able to get the secret of immortality. Come to think of it, has anyone in Xing ever opened the Doors? Have Xingese alchemists attempted human transmutation before?
Aaaand I'm going to stop there because I'd have to think about how equivalent trade and the passage fee fits into all this, and I'm starting to confuse myself. :/
...Apparently I'm really in the mood for discussion today. O.o;