looks to me like you can have 32 levels numbered -2147483648 to 2147483647 (i'll let you figure out the range if you use gamex86_64.so...like me). but there was previously an issue where levels that took more than 2 characters to represent would cause issues with !listplayers. that has since been fixed
if you want level 1000 to have the same permissions as level 999, you will have to set them the same. just because it is a higher level does not mean it automatically gets the same permissions