KRK 5-6-8 sitting together is terrible, because the spectrum of these monitors relay extremely similar, although each are more 'beefed' up versions from the previous.
KRK's are okay--I have the 5's, but I don't necessarily use them for mixing. They boost the low end way too much, and the highs too (although you can turn that down with a knob at the back).
Yamahas are pretty flat sounding, although, to me, they don't necessarily sound like they have the most depth. It seems those monitors are a bit more tailored to sound flat, like the HS-8's.
There are these legendary, discontinued monitors that are known to be in every professional studio, which are the NS-10's. The NS-10's draw out the ugliest bits of your mix, and apparently, if it sounds good on those, it will sound amazing everywhere. But these especially need to be mix-matched with other monitors, and can't really be used standalone. Though, I personally think these monitors are a bit overrated.
And of course, lastly, monitors are pointless if your room is not carefully treated and your monitors not precisely placed. As for knowing trying to figure out which monitors to get, I would suggest looking up a studio near you and testing out your mixes and professional mixes on those monitors to see which equipment you need.