TBH yes I do think it's OK to tell them that sometimes it's not OK just to talk until you get a response. I figure that there are times in life when it isn't OK to talk because you want to so it's up to me to teach that. When they're at school they can't just speak when they want to all the time, church, at doctors consults, when you are dealing with another adult/professional etc etc etc. I don't think "shut up" is an acceptable thing to say lol, but I do think that teaching them the acceptable social skills is part of our job.
If we're going to the library for instance I remind them that the library is for talking quietly and that as people are reading and studying they don't need to be distracted by constant noise. It works for the most part.

My DH works away full time so I totally get the over eagerness when they return.
My DD at 5.5 is still struggling with interrupting conversations just to say something random. She talks in her sleep and she's always loud.
When DH comes home she's in his face from the get go and since he doesn't live with us full time he's frustrated instantly. However since reading body language is not a small child's forte, I do think it's OK to say 'sometimes mummy and daddy would like to talk to each other without interruption. You're welcome to have a turn when we're finished what we're saying, but it's very rude just to speak over other people. And when you're speaking your aren't listening'. I do step back for a bit when he does get home and just let them get it out of their systems though.
I have to remind her many times a day that 'just because you say excuse me, it's not OK just to say what you want to say, we have to wait until it's our turn'.

When they speak and I'm distracted/dealing with something that I can't just give them my full attention I say "I heard you, I won't be a minute".