I'll start off saying I really dislike a homeowner telling me my business so I'd walk lightly telling this guy he should do this and that. (Suggest) and ask if this would be better.
Will this area be constantly heated? If so: Cold joint the slab with no expansion joint material. Later you will use a crack chaser and epoxy fill. Drill holes every 4' or so and install dowel rods. (1/2" rebar about 16" in length). Drill about 4" into existing sidewall of concrete. Use 1/2" rebar as a cage running both directions for the slab reinforcement, wire tied and on ladders raised into the middle of the pour. I like a 6 bag mix, no air entrainment, 4" slump with water reducer additive over a 10 mil vapor barrier plastic. Follow the expansion cuts that are in the existing slab. Since this will be a 12'x20' extension, one down the center should be enough. (At least). If saw cutting these, make sure they are installed as soon as you can walk on the slab. Waiting too long and they become "faux" decorative cut lines.
As far as the finish, if power troweled, have the guy finish the slab with float blades. he can get a good finish with these and not overly glaze the top. If hand finishing, trowel finish should be fine. Never add water to the surface during finishing.... it weakens the cement matrix and will cause a soft surface.
If you are in a hurry for this to hydrate the contractor can add a "high early" additive into the mix. This speeds hydration but it also shortens the working time to finish. (Be careful because on a hot day it really becomes a PIA to place and finish).
Take care on the cold joint. Sometimes it may require a light grinding at the edge of the existing to make smooth for the placement of the new concrete. (Ridges, raised areas) Hand float/trowel for an even finish.
If not heated: Install an expansion joint material. The foam board verses the black tar impregnated is better. The foam has a top strip that can be removed for a filler to be applied on it when the concrete is finished. ( I can't remember) the expansion joint material's name is as it may not be called foam board) It's the plastic looking stuff. It comes in a roll. Light grey or black.
In a perfect world, 16" on center for the rebar grid. Also, if not heated, the expansion joint material could be used around the perimeter. (Again, in a perfect would).
The BIGGEST thing is to have the granular fill compacted REALLY well. I don't like pea gravel because it moves around too much when placing concrete. Sand is just OK if it's kept wet at placement but that becomes tough if you have a vapor barrier installed. Crushed stone with fines is great and compacts well.
In any case, after the concrete has dried out enough to apply epoxy, I still would recommend shot blasting or at least spiral grinding the surface.
