1. How do you keep a cat inside and happy? And without tearing up everything without declawing it?
Play with them. The ideal thing to do is mimic a natural schedule. Cats wake up, hunt, eat, groom, and sleep (for a loooooong time). So the ideal play time is before a meal. That will keep them happy. As for not scratching things, redirection onto appropriate places works best. Just walk over, pick them up from where they are scratching and carry them over to the post and encourage scratching there (dragging a toy over it works well) Then let them scratch there in peace.
The first step is finding a scratcher your cat likes. It does not matter how badly you want them to scratch it, they decide if they like it. Generally you have three material choices...carpet, cardboard, and sisal. Neither of my cats like sisal, both cats like carpet, one cat likes cardboard. You have to test it out. Then you have both flat, horizontal scratchers, and vertical ones. My suggestion is get a cat tree for vertical and a couple horizontal scratchers. Most vertical scratching posts are too short for most cats, some will even tip over. If you do buy a vertical post try and get one like this:
http://www.purrfectpost.com/ tall and sturdy. When you bring home a post you'll find out pretty quickly whether or not the cat likes it.
You can also make your furniture less attractive by putting sticky paws (large double sided tape patches) where they scratch or by using a citrus air freshener sprayed directly onto the area their face goes while they scratch the furniture. That way they find it repulsing to scratch there. I've done that and it worked, you just have to respray the spot fairly often.
The other things to do is clip their nails. One of the main reasons cat scratch is to remove nail sheaths. You'll find them lying around the posts. When you clip nails these sheaths often pop off, reducing the cat's urge to scratch. It's obvious when my cats need a nail trim because their trips to scratching posts will increase so dramatically.
But honestly the most important thing is finding scratchers they love. I had to get creative once and make one out of fabric from the store because Willie didn't like any of the posts we had. It worked. He hasn't touched the furniture since.
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 kittens they stayed in one bedroom (lots of people use bathrooms) while we were gone. But that was only for a few weeks. MOST kittens, though not all, don't get into too much trouble. They sleep a lot. They don't really chew. They are litter box trained already. Obviously you'll have to see for yourself and make that decision. Some will chew wires, mine never did. With an adult I doubt you'd need to confine once the cat and dogs are friends.
3. What do you do for outdoor time? Do you give them outdoor time?
Nope. I tried taking them out on leash. With Neko he ended up out of his harness and I ended up bleeding, all because some random noise spooked him. Willie was great out on leash and loved it but started yowling constantly to go out and tried door darting (thankfully he's forgotten about it) so I stopped. Generally if they've never had it they won't miss it.
4. Food- in general is a good brand of dog food going to make a good brand of cat food?
Generally, yes. But I have a hard time recommending any dry foods for cats, wet food or raw it much healthier. Cats seem to react more poorly to carbs and low moisture than dogs do, from a medical standpoint. UTI's, crystals, obesity, diabetes, kidney disease, IBD, etc. seem to be running rampant through cats and these diseases are often (though not always) caused by or made worse by lack of moisture in the diet, or high carb content in the diet. Even if you can't feed all wet/raw, try to feed as much as you can.
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.
Shelter cat boxes are horrendous for many reasons. Depending on the style of shelter you've been to there are sometimes too many cats to a box or the cat gets a tiny box with a 1/4 inch of litter in it. You want a good three inches of litter in a box. These cats are usually stressed out which leads to loose stool. Also most of these cats are fed crappy brands of food and usually free fed dry food. This all makes for stinky litter. If you feed good food and scoop your box daily you shouldn't have an issue.
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 is a hard one. I got both of mine as kittens and didn't meet them first. They are perfect, but also the same breed so maybe I just click with Siamese cats. I don't know how much of who they are was caused by how they were raised. From working at the shelter I'd say it's VERY hard to have any idea what a kittens adult temperament will be, assuming none are semi feral they are pretty much all the same to me. The adults have much more obvious personality differences. BUT I think it's harder to gauge a cats personality in the shelter than it is to gauge a dog's (and even that can be hard sometimes). The shelter is a scary place for most cats, so they might not show their true colors. So It's a hard one. Some shelters allow cats out of their cages and I think that might give you a better idea of what the cats are like. If you want to avoid the getting into trouble, making your hands bleed stage, then go for an adult. Just be aware that adult cats usually take longer to adjust to a new house and to dogs and other cats.