I went and looked at your groups pics on myspace, you guys are hells angels or giging for the blue oyster? Either way not folks I'd want to be tangling with... yeah your wording is pretty descriptive, not a prob, might want to cut back on the smileys tho.
Anyway, what the others have suggested is all worth a try.
I still think you might be better off with a midi thru box if you can't get them chained. Some of your synths are older, they may not have midi thru, some synths, maybe yours or not I don't know, but somtimes you can re-assign a midi out to be a midi thu in their setup utilities, however you might start getting some really bad midi lag when chaining more than a couple together. The more you chain the more likely lag and other probs will start to happen, especially when sweeping the pitch bender and or cut & res knobs on the controller. A midi thru box will help prevent that problem having them parralleled and receiving the signals all at the same time and not have to hope each synth can pass that info along as easy as the next one.
Personally, I've never used a midi thru box though, and I just keep my chain down to one or two synths, or just sample them, saves me having to buy an extra midi cable as well, so I may not be the best person to ask if you run into a midi thru box issue or want to know which would work best for your setup. I'd like to get one myself someday.
It wouldn't hurt any to experiment and see if you can do without a midi thru box tho, you might get lucky and just have the synths by themselfs chained in one after the other to the other to another and be OK. I'm assuming you know enough not to confuse the midi data signal cables with the analog sound outputs that would go in to the mixer from each synth? The order of them shouldn't matter in theory, but in reality it might, only way to know for sure would be through trial and error. Just give it a shot, if it don't work one way try another order. If you get them working all together it shouldn't matter, you should be able to get them all to play simultaniously and or individually, but assigning each a difrent midi channel probably won't let them play simultaniously, unless you get midi processor too I'm thinking placed between the keyboard controllers and the rest of the synths. Having them all play at the same time could be a really big and nice hugely layered unbelievable sound, so that is why I'm suggesting keeping them all on the same midi channel and using the synth's own master volume levels (*not the velocity level*) or the volume levels on the mixer with each synth's analog output on a seperate analog input on the mixer so you can fade them in or out of the mix with a simple fader or knob turn, I would hope its an easy solution for your keyboardist to manage.
If all the synths have analog inputs, its possible you could chain some or all of them together as well and get away without a mixer even, but with 5 synths I imagine a mixer would be a lot easier as long as you got enough inputs on it to handle 10 mono left & rights or 5 stereos.
The only thing you'd have to worry about then is setting the synths up so their patches don't change banks when you change one of the other ones. That can be a little tricky, but usually doable if they have a decent midi setup utility on the synths and some manual reading.
Did I read you guys are from LA? You've got to have some professional engineers that would be able to come check it out too in your area, pay them a few bucks to come hook it up and get it all done proper, hopefully maybe a good idea too? Since half of us here, including myself, don't know what we are talking about half the time.
Good luck, and happy satan worshiping