I really love the scouting apps, I get a ton of money and items from recruitment since I'm constantly recruiting/fusing to fill out my compendium. There's one app that makes demons ask others to join with them, and you can even recruit unintelligible demons without the linguistic app through this method. I never took expert scout though, because I just don't need it.
As for negotiating itself, there's a few things to keep in mind. First is that their reactions to specific answers can vary - for example with a Dwarf if you say you respect the elderly he might yell out YOU'RE AFTER MY INHERITANCE and leave, or he might say that and continue the conversation. Don't get discouraged and you'll eventually learn what phrases demons tend to 'like'.
Second, as I say fairly often, always end talks if their item-begging goes on that long. You can usually pick it safely as soon as the option comes up, but if they're just asking for a life stone or whatever I usually hand it over. You'll be surprised how effective this is at getting them to join up.