ORIGINAL: cap_and_gown
If you want to be sure of getting the pocket, then make sure that tank division closest to the border is stocked up with support units and doesn't need to do any fighting before it encounters the NKVD regiment down on the Romanian border. You can assign both an pioneer battalion and a MG battalion to it at the start. In fact, you should use your initial set of admin points to assign pioneer battalions to all your panzer divisions, particularly the ones used to take Riga and to clear the path down to the Romanian border.
Personally, I like to use the PzK support units in the Mot divs -- otherwise they get too weak in their attacks. And it is a shame to waste their movement as purely flank security.
I have yet to see a loss against the NKVD reg at the Rum border by a PzDiv. However, one div will not secure the south end of the pocket. Your need two -- the NKVD, as well as the mot/arm units nearby and the strong inf div to the east have a lot of ability to break encirclements. Also the mot div up to the NE.
Interestingly, if you clear the hexes west of Tarnopol, you can leave the area south to Rumania almost unguarded (other than the guys at the base). One unit in the Pros area should suffice.
I do agree about the pioneers, though, -- they work well in the Pz Divs. The AFV support goes best in the MOT divs, I think.