Maybe it's a silly question but... could you say 'im Juli wir essen Fisch', or 'wir essen Fisch im Juli'?
Yes you can. All of the above mean the same thing.
But in german, the word/words before the verb specify emphasis.
i.e. it is the month of July in which we eat fish.
Just keep the verb at the second place...
Should the plural "Fische" have been accepted?
I would say no. Same as in English, you use the uncountable singular when talking about fish in this manner. "I bought fish", "I'm cooking fish" etc. no matter how many fishes there are.
Fische means 'fishes' as if referring to different sorts of fish; bass, gar, bream, etc. In this case, that is not what they are referring to.
JULY IS MY BIRTHDAY MONTH, YIPP YIPP YURRAY, I mean, HIP HIP HURRAY
Months are nouns typically referred to using their definite article. For example: Der Feburar ist kurz. in english we would say February is short, but in German it wouldn't sound right to leave off the article.