You would have to be generic human soldiers because anything else would be horrendously imbalanced.
This doesn't make any sense to me; soldiers are the strongest class in ME2. Also, see the next point.
So you'd basically be a Soldier class without any of the signature abilities, again balance.
If the multiplayer modes are co-op - i.e. you and your friends vs. hordes of husks/mercs/geth/collectors, balance isn't an issue. You can make different classes better at different things, e.g. having the engineer classes be good at taking down geth but poor at taking down varren.
It's not like the AI is going to complain to Bioware that we're blowing through it.
The idea of Multiplayer for ME literally disgusts me.
Putting aside that it's strange to have feelings this strong about a game, the multiplayer projected so far is standalone and has nothing to do with the main plot. If you don't want to play these predicted modes, nobody would be forcing you to do so.