I personally think fiber is a waste, and makes a slick finish more difficult. The fiber simply does not add any significant strength to the slab.
In my 60x60, we used about 4-1/2" of concrete and the wire mesh. I've had no problems with it, but I wished I'd had the sense, and the money to do a grid of rebar. We did use rebar around the perimeter and cages of it in the piers at the anchor bolts for the building columns ("red iron" steel building). We put keyways in the slab and did not saw it. Probably not enough keys as I learned later on, but it keeps the slab from shifting vertically at the joints.
You say you don't want cracks, but there are only two types of concrete, the first is that that hasn't yet cracked, and second it that that has already cracked. It WILL crack, you just want it to stay hairline cracks and not huge open crevases.
Charles
