I read that the Featherfin Squeaker is compatible with Rosy Barbs, you'll have to see how the Tiger Barbs are with him. He's also supposed to be OK with Kuhli Loaches.
Wounds will heal slower if ammonia is present. I don't think they increase the ammonia in the water. But adding him to your main tank may increase the bioload somewhat, so you will want to check if there is a small ammonia spike until the bacteria had a chance to catch up with the higher load.
That he is more sluggish is most likely caused by being in a toxic environment. Those high levels can cause him to suffer organ damage. I would move him to your other tank and observe how the Tiger Barbs get along with him. Maybe you want to start a new thread with this question to hear more opinions on this particular compatibility?! If there is an issue and they don't get along, you could try seeding his filter with media from your main tank to help increase the bacteria in his filter. That should keep those ammonia readings down!!! Either add some of the filter from your main tank to his filter, or rinse the filter of the main tank in front of the intake valve of his filter in his tank.
I'm so glad he is healing, you're awesome!