Hi @Michael Cutting, great questions and it sounds like you have a potentially exciting opportunity.
I am going to have a slightly different opinion compared to @Rob Beland.
I agree about being able to require them to sign leases. I am not sure if you can ask for a security deposit if they are already a tenant, you should ask an attorney. My guess is yes, but you cannot hold more than 1 month's rent as security deposit and the amount collected must go into an interest bearing account (which is .05% today) - 1 account per tenant OR you have to keep track of individual interest allocated if you co-mingle money. I do not find this to be a big deal (neither do my qualified tenants) and always prefer to collect a security deposit.
Massachusetts makes it very easy to find the security deposit laws and tenant-landlord laws online. But again, a good landlord attorney will keep you most protected and the cost of legal advice far outweighs doing something wrong and it favoring the tenant's bank account.
I personally always like to have a 1 year lease (nothing longer for residential) and security deposit for the following reasons:
1) I am theoretically guaranteed to not have a vacancy during that time and I know what my income will be.
2) My tenants know they are legally responsible for paying the rent during that time. Yes, they could move out overnight and I may never get paid, but legally they are responsible.
3) A security deposit shows 2 things: 1) they have the money to afford security deposit and rent (credit and income verification will prove this anyway) and 2) they are responsible for any damage they create and there is an incentive (as small as it may be) to not leave the unit damaged.
You may also have the opportunity to increase rents when asking them to sign a lease, of course given 30 days notice. They don't have to accept it, but it's a way you can add value and correct an under-valued rent situation.
Get something in writing, regardless of TAW or 1 yr lease.
Some people may not like or want to sign a lease and for those you will need to either begin the eviction process or have them sign a TAW agreement. I would caution that whatever route you take, be consistent to each and every tenant because if you say one thing but do 2 or 3 things, courts will not look positively at you for inconsistent practices.
If/when you need to find new tenants, be sure to have your own application, use the GBREB forms, and I've used the smartmove background/credit check online system multiple times with much success.
BTW, you will find multiple opinions of what to do or not do... neither is right or wrong, everyone has their own system that works for them, so you will need to make that choice on your own. But you will need to be sure you follow all the applicable laws - everyone should be consistent in that regard.