Kazzak and doomwalker are easily 5 manable with a proper tank and someone who can heal, and someone else who can cleanse(kazzak), and each one drops 500g + boe epics that I've been able to sell for 200-500g each
onyxia can be solod by certian classes, and is even easier with more than 1

Razorgore phase 1 is incredibly annoying to "solo". You have to pop the eggs while picking up all of the adds with a different character. The adds do negligible damage, but there's a lot. Mail+a shield or a moonkin is enough to tank razorgore himself.
Can you heal spam razorgore and use him to tank the adds while popping eggs? I don't really have the motivation to find out.

Don't think vael is so easy with less people. Him + his aura do very little damage, but you have around 50 seconds to do 1 million damage and then your tank dies. By the time that happens, at least 1 other person in your group will have died from burning adreniline, possibly 2.

I haven't done the suppresion room without a rogue, bring your dispels for chromagus, and hide behind Nefarian's throne to avoid shadowflame(assuming this still works)