i dont think they cant but are a lot more unlikely because they have a lot heavier slime coat so they have better protection from parasites, eels are the one that come to my mind but there are many others, remember because they are scalelss they dont do well with certain medications
the mandarin dragonettes don't and scooter blennies (which are not blennies they are dragonettes also) i'm sure their are a few more out there that don't get ich someone will chime in tobin
I'm pretty sure eels are fish...and they are one of the few w/o scales. Rather than looking for fish that don't get ich it is better to make sure that you put your fish into tanks w/ good water quality that are the right size for that particular fish. A good, healthy environment will decrease stress and then your fish are a lot less likely to get ich.