One question to the people who knows japanese. Should the artist name "ぷちでびる" be romanized as "Petit Devil"?

I believe that would be correct, despite the Hiragana.

"puti devil" is definitely wrong, it's either "puchi" (literal transcription) or petit (what they mean by it).

http://en.wikipedia.org/wiki/Romanization_of_Japanese#Chart_of_romanizations

puti is correct under Kunrei-shiki, which is what most native speakers use because it's the national standard

puchi is correct under Hepburn, which is what most foreigners use because of its better approximation