The idea that a protected class as defined by US Federal Law can be determined by a "consistent limit [to] be applied in general" is a categorical misunderstanding of the term protected class. Furthermore, the designation of a protected class is hardly the outcome of "(mostly) arbitrary limits".
A quick visit to the link I provided in my earlier comment shows a diverse range of peoples grouped together as protected classes
• Race
• Color
• Religion
• Nationality
• Age
• Sex
• Pregnancy
• Citizenship
• Familial status
• Veteran status
• Genetic information
These groups are not identified by any "consistent limit" but by a wide range of legislation and case law. Protected classes are defined by the judicious and careful application of expert legal opinion among practitioners of jurisprudence and writers of US Federal legislation, practitioners who include but are not limited to Senators, members of Congress, scholars, historians, judges, attorneys, and expert witnesses.
In other words, there is no way to apply a rigid set of parameters ("consistent limit") to find what group of people should constitute a protected class.
Additionally, there are many considerations in establishing a protected class, some of those being whether membership in such a class is voluntary as well as whether members of a proposed protected class have historically, culturally, and socioeconomically been subject to unjust discrimination.
Now, whether non-medallion owning contractors ought to be a protected class I cannot say, but I can say that such a designation will not come from facile calls for consistency in determining members of a protected class.
EDIT: grammar, formatting, augmentation of series