Posting interest and have the bones of a character concept(male), but need 2 things first:
Welcome Mystic! I'll try to answer your questions, but I'm no expert.
Since this is adjusted for E content, should the "First Years" be as capable as the student in the series were at the end of the triwizard tournament?
Short answer: no. This post
lists the spells first year students learn at the Junior Wizarding Academy (JWA) before arriving at Hogwarts and as you can see, it is pretty short. This post
details the information about the Ordinary Wizarding Levels (OWL's), that students take in the final year at JWA. Broadly speaking, first years (16 years old), will arrive at Hogwarts at around the same ability level as the second year students from the novels - e.g. Harry, Ron, Hermione et al at the beginning of Chamber of Secrets.
2nd is, if they really are "green" I'd like to request my character having done JWA in the US and transferred due to the storms and other previously mentioned plot trouble, since this is my first Potter RP and I'm not really being able to background off of him living in the isles his whole life.
I can't speak as to the viability of transferring - but if you have questions about life or growing up in Britain, there are plenty of people around here that are willing to provide answers or advice!