Do a search on the forums here re introducing a new cat. There's a LOT of information. It can take time. Ideally, you start slow, keeping them separated for a week or two and slowly get them used to each other's scents. Cats are smell-driven. It took me almost two months to introduce Mishi to Mitzi ... it was VERY SLOW. After a couple of weeks I could have them in the same room, supervisoed for about an hour. That was a year and a half ago. They're still not lovey-dovey, but they do sleep together and Mishi grooms Mitzi; Mitzi is just now starting to take tentative licks at Mishi.

As for the overweight, dandruff cat. daily brushing is almost mandatory. My Mitzi was an overweight ball of dandruff when I adopted her. Switching her to a good food (night now she's on Royal Canin Slim) has done wonders for her coat and she's lost 1.5 lbs. and is almost at her ideal weight. Bathing probably won't help as you're just drying out the skin and probably causing more flakes.

HTH