Code enforcement is likely to be a complaint driven system and the odds of you getting caught are slim.  

That is when someone will come along and sue you for selectively enforcing your policies.If your application asks whether or not an applicant has a conviction, and your policy is that you do not take convicts, then your answer is NO. 

They will respect boundaries but you have to enforce them.

Please be aware that simply putting clauses in the lease does not make them legal or enforceable.

We had one for our kids in NJ but the neighborhood didnt have a lot of rule enforcers and it was set way back.

There are accounting and reporting requirements that are ongoing throughout the terms of the arrangement.As a tenant/buyer, you are less at risk, but enforcing such a deal if the landlord/seller changes his mind, or dies, or sells the property While the seller bears most of the risk in such a transaction, doing business with a seller who is willing to take this risk, whether they are aware or not, should also be an indicator of future interaction with the seller.  

How do I enforce this rule for future tenants and not appear to be discriminatory towards them if I allow the present owners to have their pet?

If you don't care for the word 'forced' call it something else, but the way I see it,  the government pushed these loans by the policies they promulgated and selectively enforced. 

I work in law enforcement.

@Adam Schmidt The statute of limitations likely limits the right to enforce the HELOC.