I've yet to do any operations, but from the information I've gathered you can 1 tank it, or 2 tank it. It seems to be up to the preference of the people running it.
Just be happy though, in my guild we have 4 tanks at 50 so it's giving us headaches trying to figure out how to do operations.
8 man EV is all 1 tank except arguably Pylon. I am sure DPS can do that just fine too, but 2 tanks is helpful there. The initial trash in EV after you defeated the 1st boss is handy to have a 2nd tank, but not necessary.
8 man kragg's currently is Zero tanks on the boss.
Of those 4 50's, how many are Vanguards?