I'm not claiming to be an expert on any of this stuff, but I work in a LFS with over 90 aquariums, and I see first hand things that do and don't work. They run two sumps to filter those tanks, and a UGF in every tank. Personally, I think the system sucks. Diseases are indeed passed between tanks, although rarely. But, it makes it almost impossible to medicate anything. We can't medicate a tank because if we shut them off the system, they don't get heated water. It makes for a mess with trying to move around a HOT magnum and controlling flow in different tanks.
The other problem we run into at work is fish with different requirements. How do you control salinity, temperature, hardness, pH, etc. . . when all your tanks run off one system? We lose fish all the time specifically because of this.
Personally, I agree with the sponge filter idea. Even if you don't want to do a sponge filter, look into something like a "biotube" or tower of some kind (there's a great little one in the DIY section right now that could be scaled larger). You could hook up something like that to four or five different tanks if you used a little ingenuity. But personally, I wouldn't want to run EVERYTHING off of one big sump. UGFs work great in the store, but they need torn down CONSTANTLY because of the amount of fish waste that goes into them. Sponge-type filters would work a lot better simply because of the ease of cleaning. Pull it out, squeeze it out in a bucket, and pop it back in. You never have fish getting stuck under it, like you do with UGFs.
Also, something that's made a big difference for us recently- we bought a HOT magnum & move it between tanks. Sometimes we get busy, and it makes it really hard to get as much siphoning done as is needed. We also run into the problem that we don't NEED to change as much water as we do every day (total works out to be about a 10-20% water change DAILY), but we can't keep the gravel clean because of the amount of fish in the tanks unless we do a ton of siphoning. The HOT helps a lot- now we put it on one tank, stir up the gravel, and clean another tank manually. When we move to the next tank, we pull the HOT off and move it to another tank. We're cleaning every tank a LOT more often, and losing less fish because of it.
