While you need to stay on alert and exercise a reasonable amount of caution, don't go overboard. Turning horses out to "meet" each other before a trail ride is not at all the thing to do.
99 times out of a 100, the horses will get along.
I do have that 100th horse that is suicide to take in a group ride with more than 10 or 20 horses and I would never take that Psychotic Fool in a parade. The small groups I've ridden him with know how he can be and honor whatever my game plan might be so nobody gets kicked. In all my years of riding, this is the only horse I've ever owned that has such horrible social skills.
Point-being, you most likely have nothing serious to worry about, so exercise that reasonable amount of caution by keeping the horses far enough apart that nobody gets kicked, until they get used to each other.
The exception to that is if one the horses is a mare and that mare happens to be in heat --- all bets could be off.
I have seen the sweetst-wouldn't-kick-bite-or-squeal for anything mares turn positively evil when they come in heat. They don't hold any of that 1,100 pounds of PMS back when out on the trail. I've seen mares on the trail act worse, when in heat, than any stallion would ever think to do