Actually the requirements for MEPD & FISD are the same at every level. Areas 1-3 of Manny's post are spelled out in the charter and pretty consistent among every detachment in the 501st.
#4 so far only applies to MEPD, FISD, and BSN, and are consistent across the three dets.
Actually you're a member of the MEPD detachment even without being deployed. I suspect the confusion is that pretty much every sandtrooper in MEPD is deployed, and that something like 1/3rd of all 501st sandtroopers are deployed, may make one think that they have to be deployed to be a detachment member, but this is not the case.
Here is the language from the charter: