In a Trek probably going to be big on you in an 18" frame. I have a 30" inseam and take a 17" frame in a Trek and that is close at that. Not saying you couldnt ride that 18" but overall fit is first. You can lower the seat and you might get a shorter stem to improve the reach to the bars (assuming your reach is also shorter too) ....Really its up to you, if you can ride it then get it but I think you would fit a 15" frame just a little better or even a 17" but considering I have ridden a 21" frame by trek before (not comfortably but it was faster than walking) I dont see why you cant do it. Move the seat forward on the rails too that will shorten your reach to the bars too. Make the adjustments and it could easily work for you.
