If I was going to keep more than one arrowhead I'd do one per 50g of water, heavily planted and lots of broken sight lines. These are aggresive fish and need their space. Any other species of tankmates would also have to be able to hold their own, which could introduce all sorts of aggression problems. Another thing is they are very messy and grow to 6 inches. More fish in a 29g would create water stability problems.
As for the gravel, I do prefer finer grains for an arrowhead.
So to sum up, one arrowhead is a showfish in itself for a 29g. I did add a dozen ghost shrimp to my tank about two months ago. There's at least five that are still in there and know where it's safe to hang out. Those are my puffers 'tankamates'.