to my knowlege all hps requires a ballast but some of the smaller ones may use a smaller socket.
as per using 6 50's instead of one 400 thats really not a good idea as you would need about 12 50's to come close to equal the lumen output of the 400
so the 400 would be the obvious choice for a 2 by 3 area.
or maybe a 250 for a 2.5 sq area.