I've played against the AI a whole bunch, and while I have no MP experience, I've watched a lot of videos/replays of 5v5's so I have some general idea of how they are played. I can tell you, the two are almost nothing alike. The AI can get you used to the game, you can learn how to colonize fast, but after that, the AI plays very differently. I can beat unfair and cruel pretty consistently (although cruel drags on too much, the AI rebuilds their whole fleet for each planet you take), and yet the strategies used (expand towards them then starbase the front line planet) would utterly fail in MP.
If I were gonna play against the AI for the sake of learning how to play MP here's some things I'd do.
- Play with all settings on fastest, this seems to be a standard for MP and it takes some getting used to, since the game goes way faster with it, you have less time to react to things.
- Learn to colonize fast and with minimal ships (astroids can be done with just a colony). In MP your fleet might be needed elsewhere but you need to make sure you still expand.
- Build a large fleet early, most of your early game resources should go into a fleet and infrastructure needed to build it, not long term investments like research (some research should obviously still be done, the colonization techs, fleet size techs, and whatever techs are key/very strong early game for your faction)
- Don't rely on starbases. They work really well vs the AI, but you don't want to get used to that crutch since their uses are much more limited in MP. In the games I saw, they mostly weren't built until later except by some Vasari.
- Scout often to know where the enemy fleet is. Even the AI tends to keep most of it's fleet together so you can still scout to see the AI's fleet movements. In MP you always want to know where the enemy fleet is so you can react in time.
Again, I have not played MP, only seen others do it. These were all 5v5 skilled games, which is sort of a subset of MP (but one very popular on the forums it seems), so if that's what you're looking for I'm sure some regulars can say a lot more about it.