Without smooth talker, you can only answer two pagers. With it, you can answer four. It's important to note that if one teammate has smooth talker and the other doesn't, the guy who doesn't can answer the first two, but he can't answer any after that. Only the guy with smooth talker can. Very rarely I've heard of it messing up, but I'm pretty sure those stories were just a guy who stopped answered the page before it was finished rather than a bug.
You also have to be careful because having people freak out can cause a chain reaction as other people witness the people freaking out, and then more people witness those people freaking out. On most street heists, four civilians or so will spawn and walk the streets unless you tie them up or kill them. If you don't tie someone down, certain civilians will get brave and try calling the police. Keep an eye out for people that tend to move around a lot or don't listen very well. I think they tend to be the ones that try to play hero, but it's hard to tell.
I've found in the Jewelry heist it usually isn't even necessary to go into the back room, much less the alleyway. Just melee the one guard that usually hangs out in front, answer his pager while your buddy handles the civvies, and start running jewels to the car. Keep people in the street down and watch for guards in the alleys or civilians coming up from the back room and you're golden; no extra pagers need to be answered. I've solo'd it in stealth before with only hitting the front room; with two guys it should be much easier.
If you desperately need some cards or just want to rush through some missions, do Four Stores and bring an ECM jammer. You can hack the ATMs and get $22k from them, which will instantly summon the escape van. Sometimes he shows up before the first assault, making it an easy and consistent way to farm mods and stuff.
And trust me when I say that the game will get easier. Just ditch the starting weapons ASAP and maybe find a couple more people you don't mind playing with. Despite everyone's complaints, most of the randoms I've run into haven't been that bad and the size of my friends' list has doubled since I started playing. I'd offer to play with you guys, but I'm level 80-something.