Malawi, exciting, It is one of my dreamdestinations!!!
As for your questions:
Yes I would definitele recommend an Ikelite housing for your camera.
The choice of two ds-51 strobes is also a good choice in my opinion. That way you get TTL underwater and you will not have to worry that much about this... It is not that difficult to learn how to shoot manual strobes underwater but If you have one thing less to think about why not choose a setup that gives you TTL.
Lens choice:
From what I have hear the 105 is a superb lens... It will probably be very useful if the smaller cichlids are shy, but I guess it needs more strobepower, but two ike strobes will probably do the work?! (ask someone who knows)
The 60mm is probably also a good choice if you can get closer (nightdive!)...
Do you have a wideangle lens? In that case I would recommend you to also invest in some sort of port for the wideangle lens. I dont think that I have ever seen any good wideangle pictures from lake malawi that is properly showing the enviroment, its always "fish portraits"
Good luck with your trip. I would be very pleased if you wrote a few word about how it is to shoot cichlids in lake malawi and maby show uss a few of your pics when you get back!