Hi and welcome to Pet Talk.

Well it looks like you have the flea situation under control with getting them the frontline.

In regards to putting them together, when I brought my Mooky home, he was 5 weeks old and I introduced him to my Katie girl who was 12 years old at the time, and things were fine. I think an older cat seems to sense that it is just a kitten, so they just back away.
Then I got my Chipper who was 10 weeks old, Mooky was almost 1 by then and Katie was 13 then. I just let Chipper roam around and Mooky at Katie were fine with him.

A lot of people will suggest a slow introduction, by leaving the kitten in a cat carrier and putting it in the middle of a room so that Marley can investigate what is in it. If after an hour or so there is no hissing or growling you can try leaving the door open to the carrier and see what happens.
Good luck with this and let us know how things go.