Disclaimer: This has all probably been said already. I didn't read most of the responses.
1. How do you keep a cat inside and happy? And without tearing up everything without declawing it?
I give them plenty of access to windows and (their favorite) the glass door that opens into the backyard. They love laying by the glass and soaking up the sun. Mine have always been perfectly happy to stay inside. Siamese cats are notoriously fancy. lol My female would probably die if she had to walk outside. And keeping them happy isn't too difficult. Give them things to climb and scratch, get toys they like and play with them, let them lay on top of your head and suffocate you.... lol Plus some toys that they can play with on their own. There's lots of good stuff out there these days.
As for tearing up everything, it just takes a little bit of work. When they started to try to scratch my couches as kittens, I would just move scratching posts in front of the arm rests. It got them back to scratching on what I wanted them scratching. Then I just gradually started moving it away from the couch. Mine were never much trouble beyond that.
2. Do you just... leave them loose while you're gone? You don't have to crate them or make a cage for them? I'm so worried they'll get into everything...
When they were younger, I would leave them in the bathroom while I was gone. Now that they're older, I leave them loose. The only thing is that if I leave food out to defrost for the dogs while I'm gone, they will get into it. lol
3. What do you do for outdoor time? Do you give them outdoor time?
Mine don't get any outdoor time.
4. Food- in general is a good brand of dog food going to make a good brand of cat food?
Most of the time. Like others have said, it's important that you keep cats hydrated. They won't drink enough water. So if you don't feed raw, I would feed a good canned food or soak their dry food in water.
5. Litterbox and smell- how do you keep the house from smelling? I've changed shelter litterboxes and they SMELLED bad. I don't want my house to smell like dog or cat.
Partly diet. A raw fed cat doesn't really stink up the litterbox, and they have really tiny poops. A high quality canned food will smell a little worse, dry food a little worse than that. But crappy dry food and crappy canned food will stink up your entire house.
The other part is finding cat litter that you like. I don't like the smell of corn litter. I prefer Arm & Hammer. But that will just take a little time on your part, trying out a couple until you find something you like.
6. Pros and cons of adopting a kitten versus adult. I almost wonder if it might be better to find an adult for adoption that has a good, easy personality.
This has been covered pretty well from what I saw. I would go with an adult. You don't have the evil kitten stage where they get into everything and cause more trouble than you would believe possible, and you can find one that is good with dogs and has a personality that you like.
I have two cats, and I absolutely love them. AND they're useful. They kill just about anything. Roaches, snakes, spiders, mice... Having a cat when I lived in a college town was sooo nice. My apartment was the only one in the complex that never had any critters, I think. lol
Plus.... They're cuuuuute!
And you need one!
Or two. Or three. It's hard to stop with just one...