As far as Katie goes, I think that would be alright. We did have Tosh and Owen come back with the "sling-shot,: so as long as you write her return in full respect to that component of the plot, we can go with it. I don't obviously speak for Dawn, but be prepared to do something else with her besides interacting with Owen. I think they're planning (at least at this point) to have Owen and Tosh end up together, so while the drama would be fun, a longer-term idea for you as the player probably needs to happen prior to creating her and then not knowing where to go.
And in regards to the Time Lord, yeah, no problem! As long as it's obvious that it's only IC for him/her and not OoC for you - which I wouldn't expect from anyone, honestly
- then there shouldn't be an issue with an "offensive" character. Creates tension, adds drama, etc.