I will try and speak veeeeeeeeeery slooooooowly. :D
Beasley will battle and likely beat out Darian Barnes for the FB spot. Because Morris can back up both the FB and the RB we will likely only carry one true FB and as many as four RB's, Morris being both a back-up FB and RB.
In the Dolphin offense you can figure on roughly 450 carries from the RB for the season. If you give Brown 300 of them (which is still nearly 100 more carries than he has ever had) that still leaves 150 carries for a second #1 RB. No way in our system will the FB, any FB, get 150 carries a season.
What may be confusing is that Brown and Williams both started a number of games and split some carries that way. They were still largely RB carries however without either regularly taking on the lead blocking duties of a FB. Brown could sometimes line up like a FB because he can come out of the backfield and catch a lot of passes.
Does this help?