If Acnologia slayed so many dragons and that's how he became a dragon himself, is there a possibility that he could have killed Natsu, Wendy, or even Gajeel's dragon?

First of all, he hasn't killed IGNEEL, because igneel, still makes an appearance as at the fight between Sting to Rogue x Natsu to Gajeel, so igneel is still alive, it is also very likely the case the He hasn't Killed GRANDINE, because first of all, we see grandine visiting Igneel before wendy gets introduced so she was still alive and second she communicated with Polyruschuka after the Time-skip to give wendy the MIlky way and Sky drill, so she is definitely still alive, the only one we can't confirm is METALLICANA, but its very likely that he/she follows the same pattern as igneel and gradine in being alive. Hope i answered ur question okay

No I don t think so. Because in the manga Gajeel stumbled upon the dragon's grave & these were said to be around hundreds of years ago & also we have seen Igneel and Grandine communicating.So my theory is not

you can't say for sure,cause its still a suspense about the dragon disappearence, the clues a just being revealed ,we've jus gotta wait and watch

Actually the reason why Acnologia turned ino a dragon because he was so happy killing dragons so Zeref cursed him.All the dragon's are dead except Igneel,Metalicana and Grandine

Simply Put:
Metallicana is the only dragon who might've been killed, but Grandine's and Igneel's lives have been confirmed to be alive

Possibly not since we know that Igneel was last seen hiding in a volcano, Grandeeney is a orb of light and we have no idea where Metallicana is.