Oh, wait, you want to add 8 random letters to the array? (I thought you are picking letters from the array)
You can first add the entire alphabet to the array and then delete 18 random elements:
Array Load from JSON "{""c2array"":true,""size"":[26,1,1],""data"":[[[""a""]],[[""b""]],[[""c""]], .... }"
System Repeat 18 times -> Array Delete index (int(random(array.width)))