The Single Friend:
Is it better to seat my single friend with my parents, whom he knows well, or with couples closer to his age but whom he has never met?
The general rule for seating solo guests is to put them with other single folks or with guests they know. (Keep in mind, though, that guests who are twosomes should be seated at the same table as - but not necessarily next to - their partner.) In this case, it's most important to find a spot where your friend won't feel like the odd man out. If you're in regular contact with your friend, you could even ask him directly if he has a preference. Or ask yourself a few questions before finalizing your seating chart: Will your parents want to get up from their table often and socialize, leaving him alone? Would your friends engage him more, especially if you tipped them off? Then decide which scenario would be better for your friend.
My reception site features a main room and a smaller adjacent room. What's the best way to seat guests, since they won't all fit in one space?
To ensure that people seated in the smaller room don't feel alienated or unimportant, fill it with a good-natured group of guests who won't take offense. If your parents' friends fit that description, there's your solution. Perhaps you'll seat more senior guests there (they could well be delighted to be away from the music in the main room), or co-opt the concept of the kids table, putting teens and tweens in that spot. Whomever you seat there, be sure to make at least one lengthy visit to the room. When it comes to activities such as speeches or your first dance, ask the site manager for solutions, and be creative. Maybe you can have a toast in the main room before people are seated for dinner, or a PA system can be set up so guests can hear announcements.