[RPG] How to deal with overzealous questioning from one player

I suspect that there is an underlying issue here which has little or nothing to do with the rules. He's not "lawyering" in the usual sense - probing the DM to see what loopholes he can exploit. What I gather from the description is that he's second-guessing the DM with regards to how difficult the encounter is.

Personally, I would do two things. First of all, I would have a discussion with your players about allowing DM creativity. For most of us DMs, a huge part of the fun of DMing is being able to create encounters, plots and schemes using this wonderful medium. When the players start second-guessing the DM over such details, it takes away from your fun. You had fun creating that encounter and hoped that it would be a source of fun for the players, too. The kvetching worked against that. Be clear on this: DM's should get to have fun.

Second, I would talk to that player about what was the real trigger there. Is it really that he views D&D as an entirely closed gaming system and any deviation from the canon is bad-wrong-fun? Was he worried about a potential TPK and thought that you were expressing a mean streak? Or perhaps he's a frustrated DM himself and can't help but put himself "behind the DM screen"? He might not know himself, but hopefully you can tease that bit of information out of him. If it's the first item, then continue that discussion about creativity. If it's the second, then he'll probably get over it after you play for a while and he gets used to your style. If it's the third, you might find ways to involve him in your DMing - perhaps he can design your next big boss encounter for you, or you can busy him by arranging for the party to obtain a stronghold which he needs to populate with guards and traps.

TL;DR: Talk it out. Express why this incident bothered you, establish the parameters for your own fun, find out what bothered him, and take his concerns into account in future games.

Your first instinct-- gently talking to the player-- is a very good one. You can easily follow that up with a less gentle discussion laying out the basic idea that tabletop RPGs (unlike computer RPGs) are collaborative efforts where everyone needs to have fun. This includes not only the other players, but also you as the GM.

Your second instinct-- restricting this recruitment activity to when it "makes sense" in the game context-- is also a very good one. This, too, can be amplified beyond what you've already discussed: The idea that NPCs are just standing around waiting to be recruited is a trope in CRPGs and is sometimes a trope in tabletop RPGs but there's no overwhelming reason for it to be a trope in your game. NPCs who follow PCs around and risk their lives for them (especially when the PC is just hanging back and managing things!) are going to expect something for their troubles. It might be money, it might be respect, it might be help with their own agendas, but whatever it is, it should be something costly that the PC should have difficulty supplying.

Finally, building off that idea, there are many creative ways for a GM to make a follower at least as much trouble as he's worth, if not more. A non-exhaustive sampling includes:

• The incompetent NPC, who falls asleep on watch incurring a surprise attack at night
• The belligerent NPC, who always knows better than the boss and goes his own way
• The unpopular NPC, who manages to antagonize all the other NPCs
• The thief NPC, who robs the group
• The NPC with his own agenda, constantly steering the group into various trouble that advances his own purposes
• The wanted NPC, who has one or more groups after him for past transgressions
• The spy NPC, sent by someone else to take advantage of your PC's ardent love of followers.