Third team member for Battle Subway

I need some help building a team for the single/super single challenge on the Battle Subway in White. I already have two of my team members:

Krookodile
- Earthquake
- Outrage
- Brick Break (could change to Stone Edge)
- Crunch

Volcarona
- Heat Wave/Flamethrower
- Quiver Dance
- Bug Buzz
- Psychic

Now for my team member I've narrowed it down to three options.

Starmie - Hydro Pump/Surf/Scald, Thunderbolt, Ice Beam, Psychic/Thunder Wave/Grass Knot
or
Ferrothorn - Power Whip, Explosion/Gyro Ball/Toxic, Leech Seed/Toxic, Stealth Rock
or
Eelektross - Wild Charge/Thunderbolt, Acrobatics, Brick Break, Giga Impact/Dragon Claw/Thunder Wave/???

Starmie gives me better type coverage - especially against Fire and Fighting, and is less troublesome to obtain (I'll have to breed Leech Seed/Stealth Rock onto Ferro). However, Starmie is to some extent reduntant in its role - I already have a special sweeper in Volcarona. Ferrothorn would fill a very different role as a mixed tank. Eelktross is an alternative as a mixed attacked, though he kinda intrudes onto Krookodile's territory.

Any advice would be welcome, as to which one I should use, which moves I should select, and if you can think of another Pokemon better-suited then please, feel free to suggest alternatives. :)

Starmie - Thunderbolt/Surf/Psyhic/Ice Beam

Grass Knot is a bad choice, because Starmie isn't heavy compared to other pokemon.

Hydropump is useless, really. Surf is only 30 power below Hydro Pump, but it has 10 more PP. Lets do a check on your moveset.

Ghost Type Foe - Useful
Grass Type Foe - Useful
Water Type Foe - Useful
Flying Type Foe - Useful
Fire Type Foe - Useful
Ground Type Foe - Useful
Rock type Foe - Useful
Steel Type Foe - Useful
Fighting Type Foe - Useful
Dragon Type Foe - Useful

Go with Starmie, as it covers your weakness to water types, at some extent.
I'd probably use Thunderbolt, Ice Beam, Surf and Recover/Light Screen/Reflect.