I hear you on the savings.
One thing to consider is that an aquarium is self-bracing. Meaning, the vertical sheets of glass don't bend, so the stand does not need to be as strong in the middle. With three keggles, you essentially have three separate point loads. The stand is going to flex. Will it actually bend or break? I doubt it. But it's certainly not going to stay straight while in use.
With regard to spacing, 4" seems like a good minimum to me.
I'd consider getting your friend with the welder to extend the frame with some channel by roughly 1', and reinforce it with another crossbar and a single central leg while he's at it. Won't be much material.