Quick answer: no.
If you're a regular residential DIRECTV customer, there's only one power supply for the external SWM multiswitches, and it's the PI-29Z power supply
. You'll notice that the PI-29 is grey while the PI-21 is black, that's the easiest way to tell them apart.
The PI-21 only supplies 21 volts and that's not enough to keep a SWM up and running. (SWMs built into the dish are a little better with power) If you use a PI-21, it may work at first but you'll start running into problems, possibly including a broken SWM, pretty quickly.
On the other hand, the PI-29Z is absolutely OK to use with the SWM-enabled dish, so there's no worry there.