I agree more about multiple strains than multiple plants. I do multiple plants of the same strain and strive for uniformity - they all need the same thing at the same time.Dont go multiple plants per res is my opinion, dbkick and others on this forum (DBKICK SHOUT OUT!) have had success with it. Its just harder to examine and diagnosis multi plants in a single tote. Single res generally very successful with med to large sized plants.
In my experience there is no keeping them small in DWC.18 gal tote is 12 gal of water. I figure 2 gal per plant and 3 is better. So, 4 or 6 grown pretty large. But of course, you could do 16 plants and keep them small.