That's probably an insane level of oversizing, unless you have 500-600 square feet of "sunset view" west facing clear-glass single pane windows. Old-school rule of thumb was a ton per 500' of conditioned space (which would put you at 6 tons), but even that was overkill unless you had the leakiest uninsulated ducts running in the attic above the (crummy R19 batts) insulation layer, with no wall insulation and single pane windows throughout.
In code-min new housing a ton per 1000' usually works, (which would be 3 tons), with reasonably sealed and insulated ducts. If the ducts are inside of conditioned space rather than in a 140F attic, it can easily be as low as a ton per every 1500', which would put you at 2 tons.
If this is a new house you have the option of pulling the ducts inside, or even going ductless, if the floor plan is reasonably open. A 1-1.5 ton mini-split for the main area, and maybe a 3/4 ton for the master bedroom suite would fill bill for MANY new houses, and fix the zoning issue. And being fully modulating systems with a large turn down ratio the idle along at low-speed most of the time, which does wonders for taming those horrific muggy gulf coast latent-loads in (You could heat the place with them too, if you bought the heat pump versions, which aren't that much more expensive than cooling only.)
The only way to get a handle on the sizing and zoning is a room by room heat gain/loss calculation. The "ton per xxx" rules of thumb really don't cut it, especially if you're looking to maximize comfort. With AC, maximizing comfort will often maximize efficiency. The only reason any body would oversize by that much is if they were they type who turn off the AC in the AM, then come home at 4PM to a 95F house and want to cool it off fast. With a right-sized modulating ductless it works best and most-efficiently with a "set and forget" approach to the temperature, the house never gets torrid or sticky, and you achieve an SEER >>20 (if you buy a better-class ductless.) Most houses could do pretty well with a couple of ductless heads, but if need be there are 2-2.5 ton 3 & 4 head multi-splits out there.
Modulating ductless systems set to a single temp are nicer to the power company too, since instead of having a 4-5PM load that's 5x higher than it needed to be for the instantaneous late afternoon cooling load, the peak loads will be much lower, and occur 3 hours earlier. The part-load efficiency of these things is just insane- almost too good to believe until you've reviewed third party
lab test data.