Since I fully re-plumbed the property, I made sure that both units had plumbing lines separated and we installed sub-meters with a wireless transmitter.  

Forgot to mention that the distinction between meters costing thousands of dollars vs. about a hundred may be whether or not the meters are installed by the utility... where I'm based the public utilities charge an arm and a leg for meter installation, but we are installing our own cold water submeters (upstream of the utility meter) in each unit for somewhere around $200 each I believe (don't have the numbers in front of me), which includes the cost of the wireless transmitters, repeaters, and receiver.

Sure, I can see some expense in advertising your place for rent however its not like its a wireless carrier charging you an exit fee because they incur fees to give you a phone at a discounted price. 

As far as accessing your info via your phone, you could buy a home wireless router that has a USB drive on it. 

You also need to use wireless access points with the ability to broadcast multiple isolated networks that you can manage remotely and at high enough speeds that will provide tenants with at least 100mps each.

It is basically a wireless security camera with a built in LTE connection so you don't have to have wifi to run them. 

You already have a move in special, but it sounds like you need to be more agressive and then offer the special.If you are going young professional I would mention wireless speed and coverage too.

Hi everyone, I want to install some wireless cameras on the common areas for our first multifamily property (6 units) and someone told me that they might no be legal so I just want to make sure.

The only thing I'd spend money on is $10 or so for business cards from a place like overnightprints.com.As for other resources for frugality, your local guru is MrMoneyMustache.comThe other really useful life hack I've found is Republic wireless for cell service.  

Along with my experience in contracting in Commercial / Wireless, I've been heavily involved in marketing and business development.