Oh, I thought you had the Penguin 350. I don't know about that filter you have, but it seems to hold very little media, thus not have very effective biological filtration.
Providing lots of hiding spots probably won't help keep the kuhlis safe. The bgk is nocturnal and uses an electrical field to navigate and find food, so it will likely find the hiding kuhli. If you like the general body form of a kulhli, a weather loach (
Misgurnus angullicaudatus) would probably work. They look like a lot like a kuhli, but bigger. The eels will burrow and sift the sand though, so a sand sifter/bottom feeder really isn't necessary. Personally, I would ditch the cleanup crew since the eels will do a good job and add a bichir, catfish, or some other oddball. It is all up to you though, having a clean up crew won't be bad, but I was never really fascinated by them.
The other fish in the tank will appreciate a planted tank. The bgk is often found in underwater root systems and the
Panaque genus of plecos appears to actually need to eat wood, so I would recommend having some bog wood and some other plants in there. The abf likes floating cover, so having some duckweed or some other floating plants would be good.
When you are buying your eel(s), be 100% sure as to what species you are getting. Virtually every book gets them wrong.
This article by Neal Monks is one of the best I have seen as far as spiny eel id and care. Fishbase is also your friend here. Just to make sure, I would post pics here before you buy your eel so we can make sure you get the species you want.
What are the dimensions of the tank? If the bgk will get a different tank when it gets too big, you could probably have a few more fish in there. If the bgk will be in there for life, that is probably a good stocking depending on the number of the cleanup crew and eels.
Ryan