If you like the "real" puffers, consider a dogface. They are fairly mellow, get up to about a foot long, which should be doable in your 158 gallon tank. Tobies are also very nice, they get no more than 4", and they can be very nippy. I personally prefer the look of the Arothron puffers, but in smaller tanks, I'll put a toby.
Another puffer I can recommend is the Southern puffer, but you don't see them in the pet stores much. Mine is small, but growing fast. They get about a foot long. Great personalities (from what I can tell so far).
Here's a pic from the web:
What are the dimensions of your tank? Based on gallonage alone, if you have decent filtration and keep to routine maintenance, I think you could keep 3-5 decent sized fish (all reaching a maximum size of around 8-12"). Reading over previous posts, I see my advice is similar to that you got from justinpsmith. Guess great minds think alike!
I'd recommend:
Dogface puffer (check out the interesting color morphs too)
Australian harlequin tuskfish
Moderate sized trigger (picasso, or if you like mellower, Indian, blue jaw or sargassum)
One of the smaller tangs (mimic tang is my favorite, but kole tang or similar sized would be fine too)
You might be able to keep two dwarf angels in a tank this size, like a coral beauty and a flame (which would add some nice color). I had a royal gramma for a long time, and for three years kept it and a tomato clown with a HT, puffer, mimic tang, emperor angel and soapfish, and nothing ever bothered it.
You're going to have a lot of fun preparing and stocking your tank!
To your original question, I keep a few large hermits in my tank. They do fine as CUC, but I don't have much excess food, as everything in my tank is always ravenous.
Unless you love eels, I'd save your bioload for fish. I have an open topped canopy, and my zebra moray and Brazilian dragon moray have never wandered. However, the two snowflake eels I have had in my life each got out of what I thought were securely covered tanks and died on the floor.
