Yea but even using your method you are mistaken I think.
Sure you could use the MC34063, but it may be hard to meet the ripple spec. with a switching regulator. Have you ever made a DC/DC converter like that before?
Most likely the modules will work adequately even with higher ripple/noise than originally specified.
You really have to know the overall current comsumption of all the modules.
If it is less than 100mA like the 512 that is most easily done with linear regulators like 7812/7912. Most likely you can get away with more noise on the 24V supplies and I would just feed that directly from a switchmode supply and make linear regulated 12V from those rails. Wastes a bit of power, don't know if that is a problem for you, but you would get clean 12V rails.
Just how portable does this have to be, and do you have mains power for the source or some battery or something?
ST