Somewhat rambling post since I'm half asleep.
The unit cap is set to match up with multiplayer as everyone has somewhat smaller amounts of ships allocated to them in multiplayer games compared to a single player game. So the total number of each type of ship will usually be somewhat less then eight times the maximum number of ships if you're playing an eight player game.
N-player start worlds are a bit easier to begin with, depending upon the AI, I haven't played them extensively on anything less then 8.3. The issue with them is they get progressively harder as the game goes by, by a significant amount. Since the balance is assumed that for an 8-player world, you've got 8-humans trying to wipe the AI out, however in 8-start-world games you've got one player trying to micro a significant amount of ships.
Then there's the increased size of attack waves, I'm used to getting around 4000+ ships every 20 minutes by the 2 hour mark in my AI10 games. Plus the significantly increased amount of ships reinforcing adjacent AI worlds.
Then of course there's the problem that with you trying to move around a 3000+ attack fleet, and the AI moving around a 2000+ mobile attack force, that basically your machine slows to a crawl as well.
Generally I wouldn't recommend playing at more then 4-start-worlds. It'll mean less hurt in the long run, you'll actually get something out of the advanced science labs (since the maximum number of special ships is 8 ), and you probably won't end up ever using more then a couple of your special ships anyway since you simply won't have enough knowledge to get the higher tiers, nor the resource in the long run to maintain such a huge fleet.
One thing that doesn't scale is that you don't get more energy. In an 8 player game, each player can place a single MkI/II/III reactor on each of each other's worlds without it loosing power, however since you're only a single player, you can still only put a single MkI/II/III reactor per world, and with this you've got to try and sustain a massive army. As a result I usually completely skip the MkI series of ships since you simply can't afford to spare the resource to power the powerplants to support them.
As X mentioned, you have to protect 8 home command centres, rather then just one. This can get surprisingly difficult at times when I've had a bunch of AI ships (usually some form of teleporter or cloaked ship) slip past my main defenses and somehow manage to get to assault one of my considerably less defended planets. Usually it means I need to load the game a few seconds after I notice the warning message.
I can't seem to find it at the moment, but there should be a longish thread started by myself discussing all this with His X-ness back when he first introduced the game mode. I recall the summary of the discussion was that it's a little differently balanced, but it was supposed to play as a "variant" so it wasn't supposed to match perfectly up with either the normal single player or normal multiplayer gameplay.
