It also helps to put the classes higher in the source file than the uses of them (which matches how classes get used when in separate files). The good practices that you described are also ways to overcome not using this "natural order" but it can still be a good idea to stick to the usual order.
I'll also note that the original code smells like a typical design made after reading a typical introduction to OO programming. Jumping to using inheritance is probably the most common mistake made by OO programmers who haven't yet become old and tired. Old, tired OO programmers have learned that an "is a" relationship is very tight and quite inflexible and so should be reserved for rather rare cases and only used for a very fundamental connection (and that fundamental connections can still usually be better implemented without inheritance).