I think 18 inches seems too close together, at least for the trot. As others mentioned, it really depends on the length of stride for your horse, so you will need to closely observe and then adjust the poles as needed. You'll want to encourage your horse to lengthen his/her stride a bit. I agree with FurstPlaceMiniatures that it is a good idea to eventually familiarize your horse with a variety of spacing, because at a show it will not be set up for your horse. I was in an obstacle class recently where the poles were unevenly spaced (I don't know if this was an accident or intentional) and some of the horses had a hard time with that, but my horse took it in stride, so to speak!