Microfiber mix is usually used in thin concrete pours or added to strengthen the concrete. According to the data sheet is not a substitute for reinforcement steel. Wire mesh is recommended only in a sidewalk or patios up to 4" thick concrete don't use it anywhere you are going to drive on. For a thicker concrete use rebar. I don't really see the need for 8" concrete in your garage, normally 4" with 3/8 rebar 16" on center with a perimeter concrete beam is used. Sometimes even 6" thick and 1/2 rebar is used. But if you want to make it 8 inches well is recommended to use 5/8 rebar or at least 1/2. Good luck.
Panson is correct. Fibers reinforce the concrete but not in the same way as reinforcing steel. When load is placed on a concrete slab the bottom of the slab wants to pull apart. The tensile strength of steel is needed to resist the pulling forces because concrete develops about tens times more compressive strength than tensile strength. Number 5 bars should be placed 2" off the bottom. The depth depends on what uses you plan to make of the slab. The fibers will increase durability and reduce cracking.