Don’t know what you have so cannot fully comment. Mine is a 12’ FX50S with the big brakes S upgraded Akibono Calipers etc and other assorted things to look out for. 21’s are stock on my S. Personally, I’d roll under your car and check the clearance (control arms etc. and your calipers - crucial). But in all frankness, unless you are radically changing things, 6 mm shouldn’t impact anything but if it was my call, I’d move the offset outward a hair just in case, just me being conscious and your picking up 10 mm on the tire too BUT your not lowering (to my knowledge) the vehicle so you should be fine.
For instance, on one of the roadsters, I’ve 17x7’s high offset stock but went to 17x9’s for summer. Pushed the wheels out 6 mm to offset the difference in rim width (so lowered the offset) and studiously chose a Super Sport summer tire with accurate dimensions (Tire Rack online specs - quite useful). All together, I picked up two inches in width, great grip, great look and my unsprung weight increased by only 4 ounces per corner. So, without increasing unstrung weight, the car handles fantastically AND no real increase in weight so the vehicle stays nimble and you are not beating the crap out of your suspension. Hard to beat
For what your doing, check the callipers etc but if it’s all stock and not an S version, I’d say you are more than fine. I remind you I’m basing all this on your FX being a standard one and moving down an inch shouldn’t be any problem. I’ve actually thought about buying 2012 FX 37 or FX 50 stock 20 inch wheels for winter which I know will fit but it’s the calipers that I have to worry about. Best of luck.
RIP - Hunter S. Thompson