I don't think you should restrict the players to a Mantas or Walrus as an actual game feature. If we were to limit the amount of players in a game to five (for each team), then that means we've got a commander (perhaps selected by your method, or selected in the lobby by team vote) and four players. Then those four other players could alternate between the eight units at their own choice, depending on what they want to play or which unit is more useful at the time.
There are times where having four players all in one unit type could be useful, or split them up so there's two in each.
And if the team wants to nominate a Walrus/Manta commander, then they can do so, but there doesn't need to be any actual gameplay mechanic lableing them as such.