There are no positional requirements for the reserves. Coaches usually try to balance the rosters, but its not a requirement.
Just last year, 4 of the 7 Eastern Conference reserves were guards. In fact, all of them were point guards (Isaiah Thomas, Kemba Walker, Kyle Lowry and John Wall).
The West only had two guards (Westbrook and Klay Thompson), but had THREE centers among their 7 reserves (Marc Gasol, Demarcus Cousins and DeAndre Jordan). Selecting another guard for the West squad over one of those centers would have actually given the West a more balanced roster.
BNM