Color change is normal, could have been he was stressed in the store, or that he was not the dominant fish (darker means a happy/healthy jack), also could have been simply the gravel, light gravel will make a jack turn lighter to blend in. I cant really say how your jack will act to the other fish though as all personalities are differnt, my male jack will kill anything that is in his tank but this happened after his girl was killed, he was docile before that. My female jack only really gives chase, but for the most part doesnt care about my firemouth or severum. But, I think gouramis and angels arnt a good idea for the bichir do to that fact they are not that fast and the bichir might want to eat them when it gets big, plus gouramis are a-holes anyway.