+1 for Zigzag. Mine is super friendly and personable, and my SA cichlids leave her alone. I wouldn't go for a peacock eel cuz they have always just hid in the sand
Peacock or zig-zag definitely. I kept 2 peacocks in a 55g and they were out all the time. They don't get very big at all, less than a foot, and I had no problems aggression wise. All I know about the zig-zag is that they stay small as well, but I've never kept one myself.
Anything that will max at 12" will be fine in a 40b. every spiney i've owned i'de catogorize in the semi-agg range. give it a pvc elbow or other hide spot the cichlids wont be bale to get into as well, and it'll be fine with even the most aggressive fish. the higher up on the domincae ladder a spiney is in a tank the more outgoing ime they tend to be.