You can just use bolt tension but the dropouts will probably be out of parallell from being pulled in and so your axle nuts may have a hard time gripping. Also the two sides may not come in equally, leading to tracking problems (note that I don't know this for sure, it's just a guess). It's really easy to add spacers to your axle; if you know how it will take five minutes and if you don't know how it will be a good lesson on how your hub is put together. I'd just do that.
Your rim might be stating minimum and maximum recommended sizes. 700*18 and 700*23 go on the same diameter rim, the 23s are just fatter. 23 is still just about as skinny as you'd want to go for a bike ridden off the track---it's what most road racers use. In practice you can exceed recommended sizes by quite a lot, as many cyclocross riders do, as long as you take extra care when mounting the tiire.