Hm.. so if we made a group with some kind of tank plus those 5 classes types you mentioned, that would cover most of the important buffs?
What about debuffs on the target?
Also, which classes have the best physical damage buffs?
If you're compiling a 6-person group, having 3 healers in it would
not be a good idea. P/S is good in PvP, but its damage is severely lacking in PvE. Making a 6-person group solely based on buffs won't work. You need actual DPS, and healers are not DPS.
D/W has the best physical damage buff in the game. D/Wd has a damage buff for casters (not sure if it stacks with P/S matk buff). Wl buff is very, very good and stacks with hero potions which have a similar effect. Physical damage buffs won't help much, though, if all of you are playing casters and plan on grouping with each other mostly. You will need physical DPS to swap some people with (or to join you in a 12-man) if you plan on doing any difficult content, however, as casters just can't keep up with physical damage (exception being W/M, though it's not really a caster even if it does magic damage), especially given the increase that physical classes got with the newer instance, and the lack of an increase for casters.
In a 6 man, i would pick two healer combos maximum if you'll be together frequently. Keep in mind that there are always third classes -- a P/K can also be a P/S, for example, and a D/W can also be a D/Wd or D/S. You'll want warlocks, mages, or W/M for caster DPS.