Using Buck converter and LDO from the same power source
But I'm not so sure if it's ok to directly connect buck converter and the LDO to a single 20V,5A power supply rail.


It depends on the behavior of your supplies and of the rail, but in general that's not a problem.  

What could be a problem even with just the one converter to 18V is that if that rail is coming from a switching regulator that's does something severe like going into fault mode on overcurrent, then you need to make sure you have a soft-start on all your supplies, and possibly strict current limiting.  If you're really limited to 100W, then you don't have much overhead for inefficiency in your switchers.  If your 20V, 5A supply can't handle transient overloads, then in turn you need to make sure it never sees them.