PDZA did house Harbour Seals with their Walrus "E.T" at one point in time, however after they lost a few of the seals to walrus related causes I believe they discontinued the practice. For the most part, walruses are housed either alone or with other walrus. Due to their large size, extreme strength, and tactile nature, they tend to do best in same species groups. I actually can't think of any mixed species exhibits off the top of my head right now! Edit: Actually I lied. Seaworld San Antonio did house their young male Mitik with Sea Lions (along with their other female walrus) on and off for socialization purposes while he was living down there, however now that he is at San Diego with 2 other females the practice has been stopped. I would imagine that Seaworld's single performing animals at the Orlando and San Antonio locations probably periodically co-habitate with the Sea Lions out of shows for socialisation purposes.
Hagenbeck has kept walrusses in the past with seals and sealions, grey seals and a part of their group currently with south american sealions. Hannover Zoo has kept walrusses together with Southern Elephant Seals, Munich togehter with Fur seals and Sealions without any problems.
