Hi there SirCastic,
Sadly I think any sort of eel is a bad idea for this tank, the smaller fish will become snacks as the eel grows.
If you like the look of eels, I'd look into some loaches for the tank, like kuhli loaches. (The red tailed shark might be aggressive towards these small loaches, but I am not sure, maybe someone else will know).
Instead of adding new fish, I would add some more to the groups you have now. 2 more danios, 4 more tiger barbs would be nice. A few more swordtails for your lone one (swordtails and platies will cross with each other, so watch for that). After you bump up the groups, you still have some room for new fish.
Red Thai crab- This little guy needs land access. He is also a brackish water animal (meaning halfway between fresh and salt water). He will not live very long in fresh water (maybe a few months at most, as opposed to a few years). You might want to think of setting him up his own separate tank and getting him a few more of his own kind.
The more common name for him is red clawed crab, if you want to do some more research on him. These animals are commonly sold as freshwater for fish tanks, they really have their own needs and have no place in a freshwater aquarium.