Old cat aggressive towards new cat

I have had my older kitty literally since the day he was born, he will be 15 in a few weeks. He grew up with his mama, she passed away a few years ago. We recently adopted a six year old Siamese girl from a family who could no longer keep her, and was told she is good with dogs, male cats and kids.

We are starting week four with the new kitty and I am not quite sure how things are going. When we brought her home we set her up with food, litterbox, toys, etc in my daughter's room (I considered the bathroom but I decided if this was going to take a while I didn't want the new kitty in the bathroom for months). New kitty is a sweet girl, loves to be petted and will come out from under the bed for attention when she knows the other cat is not around.

For the first several days I kept the door shut at all times, then slowly introduced each cat's scent to each cat, let them eat close to eachother (this was actually not a problem!). So I started keeping the door open while we are at home to supervise, but I am afraid my older cat is "stalking" this new kitty, he will lay there and watch her and thump his tail. He is a pretty big boy, easily twice the size of the new girl who weighs maybe 8lbs. I am sure she is intimidating her. Also, new kitty is declawed in the front where my old man is not.

I discovered a few nights ago that new kitty gets curious at night and will come out of the bedroom and look around. So the other night I shut old kitty in the bedroom with us and let little girl roam free. When I opened the door in the morning she was on the other side and old kitty tore after her I honestly didn't think at his age he would move like that but... So now I have kind of started over. Yesterday I bought one of the feliway diffusers and hoping that does something to calm my old man down. I realize it will probably take quite a bit more time and really hope that we can get to a place where old man doesn't feel so threatened by new girl. Any tips?

Welcome to the forums.  Cat to Cat introductions can be very simple or simply awful!!  Yet, it can be done, but lots of time and patience are needed.  My resident cat was 5 when I brought a stray turned feral into our home.  The introduction period literally lasted one year.  We had improvements, set backs, improvements, setbacks and finally only improvement. 

To begin with, I would probably start introductions over completely.  I will post a few different links as well as a video with some great suggestions.  I did find the feliway adaptors helpful.  Just be sure you have a few around.  You can also get the spray. 

I found Composure liquid max to be very helpful for the cats.  It helped to calm them both down.  Then after awhile, it was the stray/feral who was more aggressive that needed it.  My resident cat was fine.  Be sure to have lots of vertical space.  It can be cat trees or just shelving on the wall.  Take a look at Jackson Galaxy's site on catification.  Playtime with the kitties is also very important.  Right now you will want to play with each one separately.  A tired kitty is much better behaved.  Give yummy rewards too.  You can use cat treats, plain cooked chicken, tuna or even Gerber stage 2 chicken or turkey baby food.  Just get the kind with only chicken/turkey and water, no added spices.  Cats love it!!

I used baby gates to block doors, but found that even with 2 stacked on top of each other, someone would jump it or knock it down.  I then bought a $20 wooden screen door from Lowes and attached it to the door with tension rods.  The cats could see/smell each other, but not get at each other.  I also did not have to deface my door.  I have a picture of the set up if you'd like to see.

Here are some links for you:

http://www.thecatsite.com/a/introducing-cats-to-cats

http://www.catbehaviorassociates.com/a-simple-little-trick-to-use-during-new-cat-introductions/

http://www.aspca.org/pet-care/virtual-pet-behaviorist/cat-behavior/introducing-your-cat-new-cat

http://jacksongalaxy.com/2010/10/01/cat-to-cat-introductions/