I hope that more zoos will focus on keeping bachelor herds of male elephants, but I am strongly against the idea to keep single bulls or pairs of 2 males in zoos which have not enough space for a breeding herd. The idea of zoos like Krefeld or Amsterdam to have a lone male in their tiny, barren yards with no space to move and nothing to do, literally bored to death, is disgusting.
Bull elephants ARE social creatures and need contact to other elephants, so the idea of keeping lone males is out. Keeping 2 together wold be better, but not good enough if you consider that wild male elephants socialize with many other males, not just one. While keeping bulls of the same age and strength together will probably not work, it should be possible to keep groups with one very large, fully mature and well-socialized male (like Alexander) with 2-3 younger males, preferably of different ages, too. That would provide a good social environment for the old and the younger bulls alike and the presence of the very large, dominant male would keep the young males "in line", prevent fights and guarantee stability for a long time (at least 5-10 years).
However, I think it is a big mistake to assume such bull groups could be housed in enclosures too small for a larger female group. You forget that elephants need exercise to stay healthy, and that they often get very sick if they don`t and get too fat - nearly all older elephants which have spent their life in small, barren, boring enclosures have problems with nail infections and arthritis. Not to mention the suffering through boredom. I find it difficult to give an exact number that should be the minimum outdoor space for elephants (male and female groups alike), but it is clear that zoos which don`t have the outdoor space for a female herd shouldn`t keep bulls, neither. Certainly not if you consider that young bulls will usually move MORE then adult females!! However, bull groups of 3-4 animals could be an option for zoos that have space, but not the money to build a barn large enough for a female breeding herd with bull barn and large female barn incl. indoor paddock ect. The perfect example for this is the Dierenrijk in Mierlo, Netherlands, which houses 3 young bulls in a simple, cheap 3-stall-barn. Such a house is not suitable for a breeding group with a big male and a growing group of females with calves, but it`s perfectly fine for 3 bulls.