ForgeEssentials was removed from the server files earlier to remedy this. You can download the latest ForgeEssential builds yourself which don't have this problem.
When FE gets out a fully stable build we will start including it in server packages again.
At this time I wouldn't recommend it. When people enter certain forums for servers they should expect it to be for that mod pack. Not a completely custom one.
Server-specific modpacks are considered private by the modding community. There are many out there. We don't want to have a custom servers section just yet because we aren't sure how we want to approach it. It may be looked at in the future.
For now feel free to modify any of the current packs and still post your servers in any of the sections. They should still be based on the main pack but you can freely add what you want. Just make sure to put something in your thread title so we know its a custom server. (As well as the link to the URL for the custom zip in the server post)
This is not ForgeBukkit, this is BukkitForge. Its still heavily under development and not very stable yet. Either way, several Tekkit Lite servers are already using it.
We updated the modpack.jar in the custom slots and the vanilla pack. If you redownload it (clear cache) the new modpack.jar in there will be able to be opened.