I thought about Gandhi as well and I might try it some time. Most cities won't grow well past 6 pop but there Gandhi already has +1 happiness over a normal civ. On the other hand, +2 culture, or +3 gold are probably a more useful bonus.
BTW, I haven't really done much experimenting, but wouldn't large populations in cities favor science at least because you can afford to put more specialists in science buildings?
You might be prone to think that but the total population of multiple cities is higher than that of a single, larger city. In the limiting case of being governed by happiness, you can have one extra copy of each happy building, so the 2 cities will have an additional happiness of +4, corresponding to a population size of 4 more citizens, at the time you depend on colosseums. An error most people make when claiming that, scientifically, larger cities are better, is to assume that a single city will have the same population as two smaller cities combined, which it won't.
Another reason is that you won't usually be in a static case with large cities. You'll always want them to grow, which means those specialists hurt. For smaller cities, I will impose a fairly low max cap on population and enforce that by giving each city two scientists. Three cities with size 6 and two scientists each yield 45 science while a single city with pop 12, two scientists and a university yields 36. I think this is a roughly realistic scenario considering the small cities grow to size six in a very short time.
In my games, I've found I always research a lot faster if I go for a massive amount of cities than when I have a smaller number.
If you did this on immortal or higher, you got ridiculously lucky. There is no way the computer will let you expand like that without a military. They will attack you starting at around 2000 BC, especially if you settle anywhere near them, and with that many cities you are bound to ruffle a couple feathers.
Who said you don't have a military? I usually start pumping out horsemen at about the time I build my third city. It slows your expansion down a bit but you can easily combine ICS into a rush strategy if you like. In fact, I often destroy one or two opponents if I feel they are too close, then settle all that nice land.