Exactly what happened when Greed hit Wrath in the eye? Did he destroy his philosopher's stone or in any way diminish his abilities? Also, why didn't Wrath use the philosopher's stone he got in his fight against Scar? Even if homunculi who were humans given a piece of Father's stone can't absorb other philosopher's stones, he should have been able to use it for alchemy, unless he was completely ignorant of the science, or whatever penalty Pride paid for forcing Mustang to perform human transmutation applied to all alchemy. In the last case, it would be important to know if Wrath's philosopher's stone was completely destroyed by Greed.

Wrath's stone had only one soul in it, due to his fighting the stone when it was injected into him. That burned away every soul in it except one. As such, he can't use the stone like the other Homunculi, so he can be killed like a normal human.

yeah, I din't understand why Wrath didn't use the philosopher's stone he got in his fight against Scar

Wrath is not an alchemist, so he wouldn't know how use it. Using the stone requires alchemy.

Was the ability to absorb any philosopher's stone exclusive to the homunculi who were never human?

Unknown. The only confirmed Homunculi are Pride (when he ate Gluttony) and Envy (when he absorbed some Mannequin Soldiers). The rest were never shown and, to my knowledge, Arakawa-san never said whether they could or not. Someone really should ask.