"Don't move! There's a sabre bear behind you!"
Translation:pevIHQo'! tlhIH 'emDaq mIl'oD tu'lu'!
Yes, that's absolutely fine. I've added this to the accepted translations, and will go through related sentences to see if both options are allowed where relevant.
Personally, I normally use tu'lu' to establish the existence/presence of something ("there is, there are, there exist(s)"), while using pronouns to describe something the existence/presence of which has already been established. So, I would be more likely to use tlhIH 'emDaq 'oHtaH mIl'oD'e'. if we were talking about a particular sabre bear, or if we'd previously discussed the fact that there were sabre bears in the area.
This isn't a hard and fast rule or anything, though.
The the/a distinction is not present here. Rather, it's more like saying It's a sabre bear behind you or It's the sabre bear behind you. It's already been established that there is an it, but maybe the identity of the it hasn't been established yet, or maybe its location hasn't been established yet, and this sentence does that.
So X tu'lu' is not really a synonym for YDaq PRO X'e'. tu'lu' talks about discovery; the pronoun version talks about presence.