If you purchased your GO 170 new it should have come with 2 fins and the 2 side fins as an extra cost option.
The smaller wide blade fin would have been the Drake Shallow water 41 (some earlier boards came with 40 and 38 versions of the wide blade large area shallow water fin).
With only a 6.0 m2 rig you could reduce the fin size to perhaps 30-32 cm and still sail the board effectively, but for full planing performance on > 7.0 m2 rigs the 38-40 cm are about as small as you can go.
Another option would be a weed fin which has more area and works much better over sandbars in shallow water as the fin will lift over the too shallow areas without hanging up/going aground and there is not the "sudden stop" that you get when a vertical fin goes aground.
As far as the side fins, yes they do help in sub planing mode and they turn the GO 170 into a low performance
beginner board with rig steering (vs foot steering without the side fins.).
The side fins are only a crutch to help real beginner sailors to stay upwind and are never to be used once your board is up to planing speed.