My unit is a single family home and when I did it last time, I was the one moving out so the way I did it was to show it to people first (of course I did some weeding on the phone call, to make sure they at least met the minimum requirements ie no pets, criminal history, etc.) but I didn't even have them fill anything out until they had seen the place.

I start by emailing them my list of requirements:1) No smokers or tobacco users2) Pets under 25 lbs only3) 2 years on the job 4) 2 years positive rental experience5) No evictions or bankruptcies6) 2.5 times rent documented take home salary7) Minimum credit score of 600.

This is a great opportunity to pre-screen them (income, pets, when they can move in, etc.) to ensure they can meet your standards.Do yourself a favor: buy "Every Landlord's Legal Guide" by NOLO.

We invested $40k on improvements.It rents for $1650/month including a $50/month pet fee.

They do make IR occupancy sensors that can differentiate between people and/pets so you could determine if an unauthorized animal is present or give a basic number of people present but I'm told they are expensive. 

after a few years she had to go in for some repair, and they had turned the place into a pet breeding operation, as hundreds of lizards, cats, dogs you name it and caused thousands in damages.

It was a non-smoking and no pet unit. 

When you speak with the sheriff, it's important to find out how many movers he requires, how full the house is with stuff, could the tenant become dangerous, are there kids on the premises, are there pets on the premises, etc.

Do you add pet addendum towards the end of lease agreement (last pages) or do they need to be singed separately after the lease is executed?

Think of all the things that could go wrong and see if the lease addresses them: unauthorized pets or tenants, early termination, security deposit, lease violations, late rent, eviction, lawn maintenance, parking, etc.5.