I can only answer based on my experience with my own, so take that for what it's worth.
1-he's almost 6 months old and very very rarely gets crazy when we're gone. He is never in a crate and roams the house freely when we're gone and when we sleep. I can count on one hand the number of times he's made a mess. And It's usually with toilet paper lol. He loves toilet paper for some reason. We have set very strict lines about what he's allowed to play with and what he's not. I suggest being super diligent and stubborn about the rules. You have to be because mastiffs are very stubborn creatures.
2- ours has been raised with 4 cats (we're not crazy cat people, I promise. I had 2 cats and so did my boyfriend when we moved in together and neither of us had the heart to get rid of any). He can be a bully with them. He doesn't hurt them but he will bark incessantly at them and chase them around the house, mostly because he just wants to play. Sadly though, they want no part of it. So he does get frustrated. But he means no real harm. However (!!!!) if you do not properly socialize them at a young age with other animals, chances are good he'll be aggressive towards cats and other dogs. Socialize socialize socialize!!
3-I have no experience with small children. We have 3 daughters, but they're all over the age of 8. I can't imagine him being anything other than gentle and protective with a baby. But like I said, I have no experience so I can't really say with any certainty. I can tell you though that just like with my cats, he can be a bully with the kids. He means no real harm, he can just be a jerk sometimes. I've had to train the kids to be assertive just as I've had to train the dog to be more submissive.
4- I have all hardwood and haven't noticed any damage. Except maybe from his drool and the ridiculous amount of slobber when he drinks water. But that's easily cleaned up before it becomes a problem.
5- Use firm, but positive reenforcement. He's going to be a big boy and needs to know that you are in control otherwise he will walk all over you, literally and figuratively. But remember that you don't have to scare or hurt him to get your point across. Doing so might cause him to lash out, which is definitely NOT what you want a 200lb mastiff doing. Food is a HUGE motivator for mine, as is a good rough playtime, so that's mostly what I've used to reward him for good behavior. I will usually just ignore bad behavior (he excessively barks when he wants attention and nips occasionally during playtime) which means I'll give him a firm "NO" and immediately stop playing with him, won't look at him, won't acknowledge the behavior at all. This has worked really well and thankfully I've been able to curb most of the bully in him.
I would honestly be worried to get a rescue mastiff. They are huge dogs, gentle by nature, but you never know how they were raised or what their temperament might be. If a mastiff decides to be aggressive, there's no stopping him. He's 200lbs of stubborn muscle. But if you have the patience to work with him every day, he'll be your best friend. I wasn't a dog person either before I got mine. My boyfriend talked me into it. I've done a complete 180 since then. He is just the best dog ever and I couldn't picture my life without him.