Oh... another thing: slew rate of the supplies? I don't know what you're getting at, but I think you are thinking way to much about this design.

As long as you have supplies with adequate current capabilibity, and you have each RS232 chip and each Cygnal device properly bypassed with good ceramics, you should be fine. (might I recommend at LEAST 0.1uF on each supply at each chip... but add a 1.0uF and/or 0.001uF if you want some extra storage/noise suppression)

Maybe someone here will better understand what you're asking, but I don't think you should even worry about slew rate of the supplies (whatever that is).