I think that it could be possible, because the author has borrowed a lot of elements from his previous work, Rare Groove Adventure. He even admitted in other media that the concept of Dragon
Slayer is borrowed from Let. The second reason that I agree with you is because Wendy noted that Porlyusica has the same voice and smell as Grandeeney.
They are also both healers and they are not human. I mean if she was why would she go around saying I hate humans. She also looked the same as 7 years before. It could be that being a dragon made her ageless which is why she looks the same. Dragons would hide because they are powerful. Their Dragon leader could want them to stay out of human affairs. If they ran around in the open as Dragons, humans would try to make them join their wars or may start worshiping them as gods.
We should stick to manga info, but this time it's kind of bewildering as Porlyusica and Grandine have the same voice actor, so why would Mashima let the anime producers make up the fact that Makarov and Porlyusica have known each other for all those years if it's wrong, while he bothers give them this clue about Porlyusica and Grandine having the same voice?