They should be able to learn skills from other classes they don't specialise in but using them comes at a penalty. Maybe less damage or % of failure. Or less distance and so on.
That or all characters are subject to universal penalties regarding this issue.
Or maybe the elves can only use no more than 2 to 3 such skills at once. And this no. could increase by 1 once every # levels like 2 to 3. And their penalties are slightly lower.. But for flesh-eating abilities,, maybe a penalty to movement, health, hearing and so on?
After all, the other glasses could get angry and riot in the forum.
