Sins of a Solar Empire has a developer .exe that lets you do all sorts of stuff inside Sins, one of them is to have an AI control the current player. The developer .exe is located in your Sins of a Solar Empire folder, alongside the regular .exe. To open the developer menu once you're in-game, press Ctrl-Shift-. (that's the period key). Once inside, to enable the AI for the current player, press [a] for Change Galaxy, then [d] for Change Player, then [b] for Enable AI for Current Player. Note that AIs enabled this way will always have the income levels of a Vicious AI and their preset is random. For the best AI vs. AI games, set all speeds except for Income Speed to Faster, set Income Speed to Slow, and make sure all AI opponents are Vicious as well. The result will be as if all AIs were actually set to Hard, including the one controlling the human player.

Not quite the same, because Vicious can buy cap ships at levels that Hard AI can't. And you probably mean Vicious @ slow-income = Hard @ faster-income. With all the changes to difficulty levels in the last few versions, I'm not sure that's precisely equality in income... Is there a more recent test of AI's actual income rates? I remember testing that a couple of years ago; see https://forums.sinsofasolarempire.com/427883/page/1/

Correct, the Vicious AI can still buy higher level capital ship crew, and the actual income rate is actually slightly higher than Hard on Faster Income (in fact, it's higher than Unfair on Faster now that I've run the actual calculations), but it's still low enough that economy management is still somewhat important for the AI, so games don't devolve into giant fleet spams instantaneously. I do know for a fact that Vicious receives 7.5x the income that Normal does. Therefore, I assume that the multipliers for Hard, Unfair, and Cruel are 1.5x, 3.5x, and 5.5x respectively (with Hard having 2.0x before the patch that knocked down their income).