Bukkit

If you can create a config file with perms and a neutral and ignore system for these aggressive mobs will do awesome.
heres a little help that the config can look like and be set up:
EnablePerms: <true/false>
  PermsGroup:
   Default: #Perm group
    Skeleton: <neutral,aggressive,ignore(or passive)> #mob
    Creeper: neutral
You should hook this plugin with permissionEx but mostly GroupManager because people use it a lot(consider that Essentials) and if you get this done I am sure this plugin will be huge! You already have ignore down you just need to set neutral and aggressive for those other neutral mobs.