could you buy an iPhone 5s or 6 on metro pcs?

No since Metro Pcs doesn't sell them but you can bring your own to them as long it's unlocked from the original provider and is compliable with their network

I would ask Mero's cust serv if the t-mobile iPhones would work , since T-Mobile now owns Metro PCS

YOu can't buy the phones from Metro, but if you get the phone Unlocked they will be able to use that phone