Lumens and number of lights

[420Paul]

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.

Some of the smaller 75wt hps lights ie, they used to use HPS for security lights, but people prefer mh because of the blue color instead of the yellow, but you can find small enough hps lights, with bulit in ballasts.