Talkgroup 017 is reserved for making contact with the SAPOL communications centre for requesting a multi agency talkgroup, you sometimes hear stuff on that one.
Talkgroup 020 is reserved for helicopter liaison, that gets used a lot for landings in rural areas.
As for a multi agency talkgroup for operations in the South East, it could be either 027, 030 or 031 but sometimes they don't follow the rules when allocating them and give out ones that you wouldn't normally expect.
If your scanner has the room put them all in, you should have 100 talkgroups per bank to play with, if you run out of space then use another bank with the same GRN site in it.
mb