No specific guideline for the template of first submission
I don't know the rules for this specific journal, but in most venues you're not supposed to tweak the page geometry (margins, headers etc.) of a template, if one is given. To me, it feels dishonest to secretly shrink the margins and squeeze in more content. It may reflect badly on you if the editor notices it.
In any case, I recommend that you contact one of the editors and ask them. That's the only way to get an authoritative answer.