I would NOT recommend using send-message-list, it tends to work in a different way of what you'd expect.
For example, any message you send with that sexp that includes a ship that was NOT in the mission when the sexp triggered, will display as an "outside" source, like command does.
Or worst, if your missions failed and your message-chain sexp already triggered, the dialogue will continue until it ends...
I cannot express the amount of frustration this provokes in me, I've seen several campaigns have this little annoying issue that for me, is an immersion breaker.
About dialogue lenght and all that, Luis and Battuta pretty much stated what I believe of this matter as well, dialogue (in missions) should stay as short and far away from combat as possible, I've found that this is a work to be done as Luis said, in an iterative way.