The one type of galvanic reaction I'm familiar with occurs when pipes of dissimilar metals are connected. Usually this occurs when iron and copper pipes are connected without the proper dielectric fittings. The deterioration can be quite rapid, and is evidenced by green colored corrosion on the pipes. Proper plumbing with dielectric fittings will prevent this. Maybe others know of different types of galvanic reactions?
I don't know of specific modeling techniques for your flushing problems, although you may be able to simply estimate the peak flow by timing the flush cycle, calculating your gpm based on the gallonage of your toilet divided by the seconds to cycle, multiplying your number of toilets, and model based on this flow. You are right that you could have a real problem, especially if you are using flush valves. Even a 1.6 gallon low-flush valve pulls that amout of water in 2-3 seconds, giving you a brief spike flow of about 30 gpm. If you had 4 flush valves get punched simultaneously that's 120 gpm. That could be a bit more than your piping can handle. I saw this exact situation once and its was causing water hammer that was traveling several miles through a 4" water main, and that was with only two toilets flushing simultaneously.
