I received the following error message when creating Parent value as IT, as per tutorial: [The value IT is not a valid number. Please enter a value with numeric characters only.]

As per your tutorial, I can tell you have used two different systems; that is why you succeeded to created a Parent value with an alphanumeric format type even though the format type for your Value Set Company is CHAR enforced with Numbers Only (0-9).

Since alphanumeric format type Parent values and Rollup Groups are more meaningful than Numeric format type Parent values and Rollup Groups, would you advise that in any future implementation, we should just use CHAR and NOT enforce it with Numbers Only (0-9), in order to mirror your implementation of Parent values, Rollup Groups and Summary Templates as per tutorial?

Yes Numbers Only option should be unchecked to mirror the implementation.