The price on the fish is typical for a store that is inland. The store I work for though sells them for $10. Online they are $7. We're on the California coast. Fish become more expensive as the fish on the market move away from the ocean.
The fish you've listed in the other post, adding the blenny would fit in fine without disrupting the system's chemistry. Take your time acclimating. Use an airline hose as a drip. Tie one end into a loose knot to control the flow of water and the other end should be anchored in the QT. Let the blenny out into a bucket with the water he comes in with and let the drip go for about an hour. Put an airstone in the bucket or swish the water around every so often. As the water level rises in the bucket, take some out and continue the drip. Once that hour is over, gently place him in the QT. Avoid putting any water from the bag into the QT. This is the basic acclimation for any livestock you get. A slow introduction to a new environment will ease the stress level of the animals and lessen risks of disease and illness brought on by acclimation and transfers from one tank to another. Fish kept in a QT, when ready to go into the main tank, should be acclimated in the same manner.
There are a lot of different hermits for the reef. The most typical you'll find are red legs, blue legs, and scarlets. Scarlets, though very attractive, are sensitive and generally get beaten up by the other hermits. They're always getting their shells stolen...LOL. If you want scarlets, wait on them.
When it comes to figuring out how much room you have to accomidate fish and inverts, instead of the rules of thumb guides of so many inches per ainmal per so many gallons of water, I go by the available territory and bio load. My little 18 gallon reef has been crammed with livestock for over 4 years without a single incident of anything dying from being over populated. Everything has it's own space and most maintain the tank. For the bulk of these 4+ years the following animals have lived quite well together (except a swap from the cardinal for the angel)...
Pajama cardinal, algae blenny, filament goby, 2 neon blue gobies, candy stripe bass, dusky jawfish, yellow watchman goby, pair of purple headed coral banded shrimp, a boxer crab, a few peppermint shrimps, skunk cleaner shrimp, fire shrimp, harlequin shrimp, several reef hermits, small cowry snail, a bunch of nassarius snails, and a bunch of coral. The coral get sold out of the tank so it's always changing. At one point in time the pajama cardinal was taken out so we could put in a tiny Imperator angel. Thing was less than 3" when she went in there. She sold after growing up a few more inches. She had all the sponge and polyps to nibble on and grew up strong. She's now living in a larger reef. The 18 is a perfect little nursery for such a tender infant stage of life for such a tender animal. You can check out pictures in my gallery here at AA. All the photos are of the 18 gallon reef. All I have on there for filtration is an Eco-System 40 refugium. The tank continues to flourish and is the most successful tank I've ever contructed, even with all those animals in a small space; defying what the majority recommend. The maintenance only requires a small water change every two to three weeks and feedings two to three times a week. Oh and clipping excess macro algae from the refugium. That is it and it's a happy tank
Reef animals live abundant like that naturally. So long as no one has to fight for a territory, are size appropriate, bio load is good and stable (plenty of live rock and a good sandbed), and plenty of oxygenation (good water flow), they'll thrive.
Enjoy your tank