You don't necessarily need a manifold. If you're careful not to fill the kegs up to the level of the gas diptube (some people even cut them shorter), you can just use a plastic T to split the gas line to several kegs from the regulator. The trouble is that if you fill up the keg past the gas dip tube and for some reason the pressure in the keg is higher than the regulator, beer can shoot into the regulator and ruin it. Most manifolds come with check valves that will prevent any liquid from flowing through them so it would save you a regulator, all depends on how much you'd like to risk it.
Also, sounds like you've got it taken care of, but if you get any more kegs don't buy the O-rings from a beer shop. Instead, do a search on here for the McMaster Carr numbers for the O-rings and buy them that way. Rather than $6 a set, you can get something like 100 of the little ones for $5-10 and 10 of the big ones for $10, saving a bit of money. Plus knowing I saved money this way, I don't feel so bad about switching out an O-ring from time to time if they start to get crappy.