I've herd the purple wolf stays smaller and tends to be less aggressive than the red wolf. I got no experience with the purple wolf, but own a red wolf. I tried keeping mine with silver dollars, severum, blue acara, and a red tail shark. Surprisingly the only one that was able to survive was the smallest one of them all, the red tail shark. But I don't swimming out in the open much. I have alot of fake plants and vases on one side and the red tail shark is able to chill there. My wolf murdered everyone else, but the severum. I was able to get him out when i saw he was being attacked.
My friend has his red wolf in a tank with senegals and a tire track eel. The red wolf will nip the senegals if they go near his cave during feeding time but nothing damaging. The red wolf and eel will also sometimes might go at it, but again no visible damage.
I personally think they're best kept alone. Mine is always out and about and begging for food when you go near the tank, while his hides a lot. After removing all the tank mates but the red tail shark I saw a big increase in my red wolfs activity.