The general rule to follow when modifying file definitions is not to change any fields in files that come with the Purchase Order system. It is very difficult to predict which fields can be changed without causing problems. There is no problem with adding more fields of your own to the end of the fields of a record.
Adding files usually doesnt create any problems. However, adding files may cause problems in some screens that are currently pushing system limits as to the number of files that can be open at one time. You just have to experiment in this area.