In real life, you have the choice with some cab companies to lease out your own cab and pay it off over time with earnings, others allow you to use company-owned cabs, and in some cases you can use your own personal vehicle if it's registered and marked according to local laws and certifications. If taxis are kept as purchasable, it should be made known that it's only for those that truly want to RP the job, and not just drive them around to flash off their special car. As long as the purchased ones stay skin-restricted the same way as the other ones, I think this could be accomplished just fine.