Hi Valken,
Purchase group is an independent enntity it is not mandatory that these group need to be only used for these PO. If a purchase group is assigned to material in Purchasing view in a material master, when you create a P.R for that material it is directly copied into P.R and when you create a PO with reference to that P.R same will be copied into PO
you can also maintain a inforecord (T.Code-ME11) with combination of material and vendor and give the requireded Purchase group in the info record . Once you create a PO same Purchase Group will be copied into the PO which is maintained in the info record you can take technical person help to make the field Purchase group for that vendor in PO not changable
Regards
RAJU