It will be interesting to see if one can be picky and get away with it for you have your advisers too and presumably need to make them happy. Since they (two at least) are romancable I assume the reverse is true that they can become rivals (even if not the same as DA2), if not leave. I've skipped headlines I've seen on Leliana due to spoilers.
If I understand correctly you also send people out on missions to take over keeps and stuff. I don't know if one can accomplish this with three or not.
I hope you're right though. I don't see me getting along with Sera too well from what I've seen of her and I don't know about Viv, for starters. After Anders, trust will be a big thing for me. But that's part of the fun as well. Bioware may surprise us and Varric will turn into an abomination.
I think the advisers are to stay, but you don't have to be nice to them.
I assume, since a party is typically three and I've not seen any more at any given time.
I missread, my bad. I don't think you send companions out to complete missions. Advisers send scouts and forces and whatnot.
Besides, I'll have a handful, slightly more than three, companions. I'm good.
I'm not big on Sera or Viv, either, but probably for very different reasons than you.
Good thing: we don't have to recruit them.