what Dirk said.
The front fins I think are very close to M5's so if you want an equal set, just get a FCS M5 back fin with a double foil.
A little caution..the fins that naish provides are high quality honeycoumb hard fins..if you buy the cheapest those are often plastic, and on the global if you really crank the turns you might break the plastic ones...thought the side fins are much more exposed, I'd still say get a higher quality fin.
If you ride the board strapless then not as needed. the softer fins does change the feel of the board...and loosens it up a little.