You don't generally subtract CR for equipment, animal companions, or mounts, so I don't see this as any different. Your characters levels, and the appropriate gear for those levels, is incorporated in CR calculations already. You don't say the PCs get less XP because they have +1 weapons, for example. Treat purchased creatures as gear because they cost money that could otherwise be spent on gear. That being said, if characters have access to more gear than they should for their level, then you would need to make your monsters tougher to compensate.
The difference between these creatures and other gear, however, is that they are independent creatures with their own personalities and motivations. Just like an intelligent magic weapon might refuse to do what the PC wants (or even convince the PC to do what they don't want to), the same could be said of NPC hirelings. If the PCs decide they want a tiger to attack their enemies, they can purchase a trained one or train one themselves (requires animal handling), but in order for the Tiger to do what the PCs want in combat, they would need to roll animal handling. They also need to pay to feed and take care of it (and spend time doing so). The same should be true for hirelings, using persuasion instead of animal handling (and possibly a higher DC because people are more difficult to convince then animals). Otherwise they will act in their own best interests. As these are slaves they might run off during a fight to try to escape while the PCs are busy in combat, unless the PCs can intimidate or persuade them otherwise. Honestly slaves are not best suited for combat. Hirelings (that the PCs pay every day) are better but still, the better the PCs treat them, the better they should obey. Even hirelings will quit if the PCs constantly put them against challenges much beyond them, risking their lives needlessly. If they mistreat these NPCs, they should get a penalty (or higher DC) on their checks to convince them to follow orders.
