I agree with feenix - it wouldn't put me off reading, but I wouldn't be like ZOMGSQUEEEIT'SROSEILUFFHER. Rose coming back stories always irk me somewhat - it's been made abundantly clear that she's trapped in this altreality, although having said which I'm sure you could do it justice.

You already have Ianto there who has the potential to cause problems for the happy couple, and I can see Jack being the cheater (or at least more easily tempted) than the Doc anyway. Based on what we know about the character, it's surprising Ianto's managed to keep his feelings for Jack hidden as long as he has in this series.

But I'll read anyway, whatever you do :D