well i'm going to be purchasing a set of gunmetal rims...17x7 with an offset of 42mm...i'm fairly new to rims and tires and i need some advice on a set of rubbers(tires)....i live in sourthern cali so snow isn't an issue...any help or advice would be appreciated..

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That offset is minus 18mm from the OEMs. What that means is you have extra clearance from the front suspension arms, and significantly reduced clearance from the rear fenders. Even if you were using the OEM tires, you might have problems at the rear unless you roll the rear fenders.
As it happens, using OEM rims (not the minus 18mm you are going to use), I purchased two pairs (quite intentionally different) tires all at once last month, and the choice that I made also may address your potential rubbing issue. I purchased Yokohama ADVAN Sport 205/55R17 tires for the rear and Goodyear Eagle GS-D3 225/50R17 tires for the front. The overall diameter of each of the four tires is identical, but the rears are
slightly narrower than the front; that is, the configuration is a "reverse staggered" orientation. (The measuring rim for the Yokohama ADVAN Sport 205/55 is 6.5", so, mounted on a 7.0" rim, the actual section width of the rear tires is actually over 210mm; the actual
contact patch of all four tires is the same as long as the air pressure of all four tires is the same.) The slight narrowness in the rear helps (but only in part) the possible rubbing issues you will have using 42mm offset rims.
The Goodyear Eagle GS-D3 has a higher treadwear rating than the Yokohama ADVAN Sport, 280 to 180. However, as Mazda6's have a strong front weight bias, most tire wear occurs at the front, and the front Goodyears likely will wear out faster than the rear Yokohamas.
Mazda6's are prone to understeer, and the "reverse staggered" arrangement described here slightly counters the understeer -- not a huge amount, but a change in the right direction. I intend to start a new thread to discuss this aspect sometime when I get a Round Tuit, but I give you a preview here.