I had just completed this model as well.
I changed the rims from a diecast model. The one from the kit is 15" and mine is 16".
However, I had also encounter the unfit problem of the rims. At first I modified the diecast rims post at the back side to match the original one, and then when I try to dry-fit the kit, I found the wheels are sticking out of the fender too much. I thought it was my diecast wheel problem so I put the original wheels on, and it was same.
And if you take a look at the official Fujimi page, their finished model wheels are sticking out of the fender as well......
As I was hurrying for a local model show some days ago, I don't have enough time to have further modify. To make it simple, I throw away the rotating brake disc as I think nobody can see it through the multi spoke rims(LOL!!). Wow. The wheels (either original kit one or my diecast one) fit perfectly now.
So I think the engineering team had forget there is a material thickness for the brake disc during they develope the wheel???
Not only this. After painting, I assemble the kit and found the fitting was way too tight. Especially the windows and side signal light. There is no way to snap them into the molded openings. Now even you dry fit an unpainted kit you will find the fitting is pretty tight. I think they did not take care about the added thickness after layers of paintings and top coatings. Molds were build tooooooo tight fit!
BTW below is few shots of my Fiat 500 from the model show:
