Thing is, if they keep the retainer system (I haven't had time to go look and see where that is in the grand scheme of things) then extra characters really aren't necessary from a mule standpoint. That's what retainers are for, mules. Since they're implementing an auction house I don't know if they're going to have them still or not, but I don't remember reading anything that said they were getting rid of them.
As far as I know, this is mostly all Yoshi has said about how the retainers are going to work. This is from the 4th live letter from the producer Q&A.
Q: What will the new market system be like? Can you give us some details?
A: Basically we will be totally re-making retainers and the market system from scratch.
We’ll be adding all the Auction House features people are asking for, and make it so that only your own retainer and those you want to appear are displayed in the market wards. It’s a bit difficult to explain. It’d be better to have our players try it for themselves in the Beta, the same time we run server stress tests.
We will also be fixing the system so that you can search for materia-enhanced items, and tell whether or not an item is HQ. In the Beta, we’d like to see how much stress the search feature can take, so please search away.
It sounds as if when you go into the wards, only the retainers who are selling the items that fit your criteria, after you've filtered out the useless stuff, will show up. I thought I read somewhere where there is a special place within the cities that retainers will also show up just like other characters, but I don't remember why or if it'll actually work like that. Maybe it's just like those bells that we used to use to call our retainers at that were scattered throughout the cities.