I was thinking of (maybe) doing of a speedrun on inFAMOUS 1, but I just want to know if it is okay if I use RPCS3 (PS3 Emulator) to speedrun the game because I don't hear a lot of inFAMOUS players in this Speedrun community talking about it.

You can do speedruns in any way you wish. Whether emulator runs are to be included in the leaderboard is a question that has not been considered. If you wish, you may join the Discord and be a part of the conversation about it (I have just posted your message there). If not, I will update this thread when a decision is reached.

Has it been decided? (I don't use Discord)

Why not?

And no, not really. These games are still at the point where emulation is incredibly tricky. I can confidently say that if emulators are to be allowed it'll be a separate subcategory.