Internet says 18.5v for 2009 models and earlier, is that right?If both PSUs put out the same voltage but one is lower wattage, then you are talking less current. As long as you do not trip the PSU's overcurrent protection it should work for testing purposes. If anything is going to die it would be the PSU, unless it failed and killed the mini as well. A good PSU should be less likely to kill things attached to it when it goes pop.
If anything at all, the mini might *might* be smart enough to throttle itself when used with a lower wattage PSU like IIRC some ancient ThinkPads have been known to do.