This still confuses me. I’ve never heard of using the term “roommates” to signify a group of people who individually rent rooms, if none of them knew each prior nor who have any relationship to each other. That sounds like a “rent a room” arrangement (which is different from being someone’s roommate or housemate), or it sounds like a boarding house. I don’t think you get much say in who rents the other rooms if you’re renting a room in a boarding-house arrangement.
Did the landlord find you all himself? Or did someone in this group of five current occupants find the other four? I don’t know; it’s just that it is not customary in any place I’ve lived for a landlord who doesn’t live in the group to find various random tenants, put them all together, with each having their own separate lease, and call them “roommates.” Maybe this is common elsewhere in the country, but having never come across it I’m having trouble understanding.
You have no grounds for asking for a reduction in rent, what the landlord may, or may not, have said means nothing. Even if he admits he said it, it’s still not binding, he can simply say he changed his mind. If you had convinced the landlord to include that in the lease, you would have some recourse, but it’s not there and it doesn’t apply. There may be some legitimacy in asking for more storage space, to replace what is lost by his renting the sixth bedroom, and in pointing out that the refrigerator is already inadequate for the number of tenants. These could be considered habitability issues, but he has no obligation to lower the rent. He’s trying to maximize his income because it’s a business, that’s what businesses do and reducing your rent would be counterproductive from his point of view.