Don't stress because I think this intro will go very well. They are both young and that really helps. I'm sure Sophie will do some of that adorable little kitten hissing but she will find Tito to be a great play buddy in no time! The older the cat the more difficult the introductions I think. Tito will be in a new place so he won't be too territorial yet and Sophie is such a tiny baby that she will just be freaked out for a short time. I think you are going to have a lot of fun with these two!
If Sophie has had only one accident in seven days I think she is doing remarkably well! She is so tiny and has such a tiny bladder that I'm sure things just kind of hit all at once! Know what I mean?!) If she is doing that good now I think your problems are almost over with the accidents.
I can't wait for more pictures of the two of them!! You are going to have many many laughs and so are we!! There is nothing cuter than two youngsters getting into trouble!