And they are looking at taking possession in about a month.

Why wouldn't the owner just push back the close date or let you know they would be there when you took possession and then move out in a month? 

You should complete the eviction process to be entirely sure you regain possession legally.

Not for their personal possessions but for the damage they caused.

It also makes it one step easier to evict them (if that becomes an issue) than with a standard lease agreement.Regarding your tenants, just have them sign your new rental agreement the moment you take possession. 

If the tenant has lived in more than two places within a two year period, it may indicate instability so I would require a previous landlord reference and make sure he actually owned the property.Whether or not the tenant's current or former landlords release information, we landlords are responsible for who we allow possession to our properties.Hope this helps!

I actually think it would not be more than a few hundred, not so bad considering the monthly rent is $3200, and I could in theory charge it against the security deposit or directly to the tenants (at least I think so).I am thinking that I should allow the 4 months to finish, get the tenants moved out, regain possession of the property, and then if necessary due to needed repairs let on that I knew about the 6th tenant, as a point of leverage.

Does anyone know if I can add a term to the agreement, to have tenant responsible for pest control after certanly weeks of taking possession of property?

Don't require it but clearly state in lease that you are not liable for renters possessions and recommend renter insurance.

You may end up having to evict him to regain possession of the property.