Yep - aggression is an issue - but a good balance of easy maintenance and other factors against some drawbacks is what you have to consider. You can easily mitigate some of the aggression issues.
I've run a QT
tank (20 Gallons) also, back when I was stocking my 125. It ran for about a year with a Niger Trigger. He was quite happy there for the duration and very easy to care for (he was about 2 inches and did little or no growing in that year). Despite some opinions, these fish are pretty peacful. See the post above and read the comments on this fish (nearly every one indicates how peaceful it is and how well it gets along with other fish) especially if you have some place for it to hide.
Also - what I would do at times of adding a fish to the qaurtine - I would simply put in a plastic opaque divider (you can buy these pre-made or be creative), leaving the trigger on the side where his PVC
hiding place was.
Got this approach from a friend who used a Panther Group for the same purpose but I must admit that fish was bought small, grew pretty fast and the friend had a 29 Gallon QT
tank. He eventually traded him back to the LFS
If you chose Kevin's advice, careful with the blennies as some are aggressive and some are venomous as well (check current literature) especially in a tank with simple landscaping. Most are moderate to difficult to keep. Don't know about the hermits or others but perhaps medication is an issue with them. I.e., What if you are required to medicate the tank?