Your six line should be fine in your aquarium. Nano Reef has stocking guidelines and, while they may be considered controversial by non-nano reefers, they are what I follow for my tanks (I consider my 40B a nano), though I've got one fish in there that would get me lynched by them.
http://www.nano-reef.com/fish/?fish=3
Understand that when they indicate that a six line can be housed in a 10 gallon, it is the only fish in there. I think they recommend four fish at the maximum for a 20 gallon.
Garlic does seem to be a popular display system treatment, and despite the lack of scientific evidence that it works, there is a huge amount of anecdotal evidence that it does.