Pet rent, deposit, fees, etc - whats legal in Mass?

[David Quinn]

Hi, 

I have a 4 plex in Marlborough.  I have a rule against pets but 2 of my longer term tenants have cats (without asking permission).  I've let it slide for a while - I know I should  have addressed it right away.  I'm thinking about giving them the choice of finding new homes for the cats or keeping them and paying pet rent.  Is this legal in Mass?  What do others do?  

Thanks in advance!