I'm sorry and I really don't mean to be discouraging or non-supportive but....Scooter doesn't stand a chance of surviving in that tank. No where near enough rock to support pods, 2 other fish that will definitely out compete it for food.
We have a responsibility to provide a safe, healthy environment for all fish, if we are going to keep them.
Just because it came from Petco isn't an excuse. Someone with a much bigger, more established tank could have boughten it also.
This is actually one of my favorite topics to share about. Its not necessarily hard to get them eating frozen foods, its more so daring yourself to let food get all over in the tank. Im not saying to dump excessive ammounts of food in there. But leave a circulation pump on to get food towards the bottom. Dragonets are slow, their eye sight is different from every other fish. If you watch them closely, they dont have a long line of sight. Just what is close to them. I have a red and green mandarin in my 65, and they both chow down on frozen foods, whether its brine or mysis. The main thing is just letting get near them to catch on. I recently just put a spotted mandarin in my 20 gallon, and the second day of having him he already goes for the frozen foods. Theres no need to put them in a breeders net or anything to try and train them. They just wont come to the top for food like everyone else. And moving them will just stress them out. Ill try and make a video for youtube on it here soon. But i have not had any issues getting mine to take to frozen foods. Nor have i had to actually "train" them to eat it. When i feed my tanks, i turn all pumps off except for one circulation pump, and thats just to make sure the food gets to the bottom. Now, i know a lot of people will argue this, but, if you have an adequate clean up crew, enough coral, and know your other fish will run around grabbing every scrap they can, its not at all a bad thing to let the food be sent around the entire tank.
And normal behavior for scooters and dragonets is to make sure they are staying active by moving around the tank and pecking at the rocks and sand. If they are just sitting in one spot all day, then sadly to say its probably too late for them, but isnt always the case. Another reason i dont mind letting food go all over the tank is cause i rarely target feed my corals. The corals will catch more food than you would expect. Then I'm not putting too many nutrients into the tank.
One last thing, the tank you have is actually very good to get them eating frozen foods. Its small enough to get foods down to the bottom without over feeding the tank. I first realized how to get mandarins and scooters to eat frozens in my 14 biocube. I had a scooter blenny and a spotted mandarin in the 14, and they were always fat and happy healthy. This is just an area that not many have experience with, and has in some ways been detoured from because of not knowing what to do.
I've only tried this with a spotted Mandarin, but worth a shot go to an Asian grocery store and get red masago eggs it's frozen caviar I used that mixed with brine and it slowly started eating brine.