Re: 225 to much?
It all depends on what the boat is rated for. If it has no rating plate and you can't get one from the manufacturer then you have to go by the standard USCG formula. There are lots of 18' boats rated for 225. Or more.
As for a Gil bracket on an 18' boat, that might be a problem. The OMC style Gil bracket has about 26" - 28" (or so) set back. I'd think that would screw up the balance big time. Again, it depends on what hull design you have.