There are variations, yes, but I'm not sure if all of those variations could be deemed acceptable.
Myself, I did a small project for myself to analyze the etymology of the names of all 493 Pokemon, and then decided where to go from there. (I've saved it in a PM; if ever I want to reveal it to the public again, I'll make a new topic.)
Arceus is one of those names that cause a lot of debate, but since the English name comes pretty much directly from the Japanese (Aruseusu), I say 'AR-see-us', regardless of what others say. (On that same note I also say 'reh-jee-ICE', and not 'rej-ice', due to his name also directly from the Japanese, 'Rejiaisu'.) Other things I say because I'm used to them, or because there are alternate possibilities (for example, I am one of the few people that pronounces Sceptile 'SKEP-tile', as I draw my etymology there from 'skeptic' and not 'scepter'. (But I list all the possibilities in my project, so I let people decide for themselves.)
...I'm not sure why that ended up in sounding like me bragging. I'll end it on a lighter note.
Right now I say 'GAH-ruh-dose', but I probably should be saying 'GYA-rah-dose'. Also, 'VILL-uh-plume' instead of 'VILE-plume' is by no means acceptable
Edit: In regards to Rattata, I prefer the latter.