Is it completely necessary to use the "ni" particle after "gogo 7 ji"?
Ni particle indicates a specific point in time, so yes
Is there a word order in japanese(aside from the verb at the end)? Or is it irrelevant as long as the particles are indicated? Becuase sometimes I feel like the word order should be correct, but it isn't.
There are certainly some rules. It is not entirely deliberate. Just like how we learned to say a big red juicy apple rather than a juicy red big apple. Don't bother too much at this stage.
Would 晩御飯 instead of 夕飯 been ok here?
does this work? 「七時午後で食事をたべます。」
It is always 午後七時 instead of 七時午後.
食事 means "the matter of eating". We do it rather than eat it. So we can only say 食事をします or 食事します.
In Nihongo, the time of the day is always used first. So it's always either 午前 (AM) or 午後 (PM) first— then the hour and minutes.
午後四時 (4 A.M.)
午前九時 (8 P.M.)
Hope this clears some things. :)