Sorry to tell you this, but your Dragon fish is probably not going to live under these conditions, hiding in a shell all day and not eating. I can just about bet you 100% that if the GSP is nipping at the crabs, it is nipping at the Dragon and that is why Dragon is hiding all the time. Maybe it has only happened a time or two, or just at night when Dragon tries to come out and eat and you just haven't seen it happen. Either way, these two fish are not a good mix.
You have 3 tanks, a 25 g and two 10 g's, and you are considering purchasing a 50+ g. This is my recommendation for you at the present time to successfully take care of the fish you have right now. I do not know the size of your Dragon but I suspect if you just got it from WalMart it is around 4" or smaller. I do not know the size of your Pleco, but under the same assumption, I am guessing 3-4" or smaller.
Use the 25 g for your Pleco and other community fish.
Use one 10 g for the GSP - acclimate slowly to brackish water
Use one 10 g for the Dragon - acclimate slowly to brackish water
If I were you, I would start here before you dive into adding any more fish. Get the fish you have now in healthy environments.
I can tell you want to take care of your fish or you would not be asking these questions. Also, there is a huge difference between the amount of salt in brackish water and salt water (Clown Fish are Salt Water Fish) I advise you use "Instant Ocean" for the brackish fish, definitely not Aquarium Salt, Kosher Salt, Table Salt, etc. You will also need to get a Hydrometer to measure the amount of salt in your water as you are acclimating the fish to brackish water, it's just a little clear plastic measuring device - costs a few bucks.
If you are seriously wanting to do this, I can provide you with website instruction to take you step by step along the way. It is not nearly as difficult as keeping a salt water tank, just a few extra steps than what you are doing now to take care of your freshwater fish.
You will eventually need larger tanks tho, this is just a quick temporary fix! Hope some of this is helpful, let me know if there's anything else I can do. Thanks!