DK and Pally group questions

[ragawaga]

I've been reading the posts of people using 5 ret groups or dk and 4 rets and it has me interested. I currently have an 80 decently geared prot pally. So would like to keep him as tank. So my questions in no order me importance are

I see that a dk is used as a tank and wondered why this is given that the pally tanks take less spikey damage?

What combo would be better of 1 prot pally, 1 ret, 3 dk ...or 1 prot pally, 2 ret, 2 dk?

Why use a dk at all? Do they bring something extra that would be lost with 5 pallys?

Slightly OT, how many levels can a level 80 grant to a linked account? Still only 30? I ask because only 2 of my accounts still have raf..