You could also just englobe them.
In the old threads, IIRC, one of the devs said the AI was optimized for medium conditions. That is, medium size galaxy, habitable planets a notch or two below the most abundant settings, one or two AIs below max, Minor AIs, two or three below max, etc. Anomalies also need to be set in the middle.
The AI has trouble with either an extended colony rush or a too short one and, as you pointed out, with galaxy sizes both so large that it makes handling fleets tough and so small that influence effects cripple its colonization logic.
The devs assertion was that, if you want the toughest game, play with all conditions somewhere in the middle and set the AI at least a couple notches above tough.