So first of all, I really like this idea. I tried it once or twice a long time ago and I'm still fantasizing about it. With that out of the way, let's move to my 2 cents.
Make the NPCs well-rounded characters
These NPCs aren't just some recurring characters, not to mention some one-time ones. They're gonna be with the party for a very long time. Whole adventures, actually, which is quite a lot. This means that they should be really well rounded. Your players should get to the point of distinguishing between them after just a few words, especially when the NPCs are the ones who spark the conversation.
More than that, though, they should be more than cardstock characters. You want them to express feelings, you want them to have goals, and you want them to react. Otherwise, your players will have hard time connecting with them, and this is far less good. As a rule of thumb, make them round and distinguishable from each other. A wizard is different from a fighter, true, but we want a greater difference. The prince will have hard time adjusting to the wilderness and to the bugs in his bed. The merchant will always stop to collect the better loot so she'll be able to earn much from those bargains. Classes are a dirty way to distinguish, but they're better when accompanied by some other traits.
Don't let them speak right one after the other
The players aren't coming for the game as an audience for a theater, they're here to play, and hearing NPCs talk with each other is far less fun. If you must make them speak between themselves, make it as quickly as you can and immediately move on. If you can narrate the conversation instead ("they're talking about what happened, Elsa thinks that they should go east and Hans thinks that they should go west…"), it is far far better.
Give the players the center stage
The players and their characters are the real stars of the campaign. The NPCs are extras, and should always be seen like for you that when you're playing them. If they're far cooler than the PCs, or if they have much more screen time, something is off with your campaign. It's better to not have the NPCs with the group than to let them illuminate and shadow the PCs.
They're not all knowing
Think for yourself what is more important, a character who knows everything or a character who knows only part of them. For me, it is the latter. That comes from one simple thing: There's no drama when everyone knows everything (it is not entirely true, but that's for another time). Make them say sometimes stupid or idiotic things, make them come to wrong conclusions, let them make mistakes. They're not a kind of supernatural deity who knows everything, but humanoids who are as humane as the characters, and they should be played this way.
Make them important for the story
They should always be important to the overall story, in one way or another. In one of my more successful D&D campaigns, the characters had to escort a princess to a neighboring kingdom through the forests. Having to keep her safe from one hand, and dealing with all of her complaints from the other one made the game so much richer. They don't have to be important to each and every one of the scenes, but they should always be important for the overall story. Otherwise, the characters may just leave them to rot one day, when things will turn the wrong way.
And an end
Hope I succeeded with helping you a little bit.