1. Good AI?
It's *pretty* good. Like any AI in any game, eventually you will find a pattern in how it behaves and will be able to exploit it, but for an RTS the AI is, I would say, pretty great. It actually was made easier in some cases due to player requests. Originally, for example, the AI was a lot more careful with its fleets. It would rarely attack a superior force head on, and would wait and attack poorly defended planets instead. But players didn't like that they couldn't get big epic fights because the AI would just retreat if it knew it was going to lose (even though players would do the same thing), so now the AI tends to throw away its ships a bit more often.
Because it is real-time AI, though, don't expect it to be up to 4x standards. In 4x, all the number crunching by AI can be done during the player's turn, which gives it a huge amount of time to figure out the next move. In an RTS like Sins, the AI has to continually analyze the situation, which cuts down a bit on the complexity of its logic.
2. Can you easily change the speed of things in Single Player mode? (I mean, slow things down for an old man like me, speed them up in the boring parts)? Pausing ok?
Pretty much everything has its own speed setting. There's an overall speed setting, there's research speed, culture speed, so on so forth. You can tweak them independently and come up with a scheme that works well for you. In Single Player, there's also a "speed up time" option (x2, x4, x8). Pausing is perfectly fine, and you can queue up/issue orders while the game is paused.
3. Is thinking good strategic stuff more important than being fast?
Yes. Capital ships and support ships have various abilities, but in the majority of cases you only need to worry about capital ship abilities. Fights in Sins can last a very long time (think 5-10 minutes for big/huge sized fleets), so you get plenty of opportunity to use important abilities without being rushed. Being 5 seconds late on using one isn't going to make much of a difference.
4. Anything else you may add is welcome.
Only that you should grab the Trinity pack. Don't bother with the base game if you somehow find it somewhere. The mini-expansions for Sins were very good, and worth every penny.
