Ish Question?

I have a Kole tang that has been doing great (or so I thought). I have had
him for about a month and a half. I noticed over the last few days that he
has what looks like two small slices(one on each side) on his tail. Is this
normal? It didn't seem to bother him and they look identical, so I thought
that maybe this is just the way he is. I have not been able to find any
pics that show clearly whether or not this is normal.

Suddenly today (only about an hour ago) I noticed what looks like ich all
over him. I want to get him treated ASAP, but I do not know if what I want
to use for a quarantine tank is sufficient. I have a small 6 gallon tank
that I have not used yet at all. It has built in light/bio-filter/heater.
Is this too small for this purpose?

In my main tank I have live rock, shrimp, snails, feather duster, green
mushroom coral, green polyp coral. I don't think I can treat this tank from
what I have read.

TIA
....D