If translating this from English, can't this be both present or past tense?
Yes, but be aware that English present tense is far, far more likely to be present continuous: "are reading a letter."
Are there indefinite articles in Esperanto?
Nope, no indefinite articles.
Aren't both "Li kaj ŝi legas leteron" and "Li kaj ŝi leteron legas" correct? I thought the word order was unimportant in Esperanto.
the choice was "La kaj si leteron legas"
I'm going to be the noob who says "Why isnt it sin no si?". Sorry. IDK why