Because a GT will most likely terrorise an acara.
No to cory's, although its said they give off something which prevents larger fish from attacking them, i wouldnt risk it, Ive just moved all of mine from the tank with the GT.
Bristlenose might work, but again moved mine out and put a common in.
Dont know about loaches, never kept them and dont know their sizes.
TB's/ Flame tetras, could be killed.
Definately not keyholes, far too timid, GT would make light work of those.
Wouldnt recommend any cichlids smaller than a GT, perhaps Firemouths.
Again, Ive moved all my rainbows out as they arent the smartest of things to act as dithers.
Giant Danios are your best bet for dithers.
Plants more likely to be uprooted, you could pot them though.
You could get a placid GT, they are not all nasty and aggressive, but you wont know that until its matured.