Translation:We are his guards.
we are the guards of him is incorrect?
That's a very strange way to say it in English.
Would female form be "שומרה" or "שומרת"?
Are the word endings for guards (to guard something) and a guard exactly the same?
Yes, in present tense for a single male
Is אנחנו "הם" השומרים שלו wrong ?
Yes, הם means they or they are
You don't need a copula after a pronoun like אנחנו
In Biblical Hebrew this was possible: אַתְּ־הוּא הָאֱלֹהִים (2Sam 7.28) You are G-d or אַתְּ־הוּא מַלְכִּי (Ps 44.5) אַתְּ־הוּא מַלְכִּי You are my king, but in Modern Hebrew never put the copula after a personal pronoun as subject.
we are his guares