To answer OP's question...based on what I've learned from customer service, you can't make "tab installments" (except for at the time of activation or upgrade). When you activate, you have the option of paying for part of or the entire balance. Once you're on a tab, your only options are to ride it out or pay off the entire remaining tab balance.
Advantages of a tab small allows you to go on a cheaper base plan (tab small plan requirements are less than tab medium plan requirements) and to have a lower bill. Once you start the tab medium, the only way to switch to a tab small is to do a hardware upgrade.
To be fair, no other wireless provider will let you pay part of the buy-out as a one-time installment after the agreement starts.