I think you need to purchase one BayStack 450-24T (24-ports 10/100 Mb) and one BayStack 450-12T (12 ports 10/100Mb).
Both can handle STP.
Thay also have a MDA slot. In this slot you can put a 4-port 100 Mb UTP module. (or 2 ports 100Mb fiber, or a Gigabit port.)
So if you need exactly 16 ports / switch, you need two BayStack 450-12T and two 4-ports 100Mb tx modules.