Certainly sounds like the best plan. New females would certainly be desirable to even up the sex ratio.Depending on the size of Monarto’s exhibit, it may be possible to integrate additional males; but it makes the most sense to add additional females from Sydney/DDZ/Overseas and breed until the desired population is reached.
It may be wise to preempt an unbalanced gender ratio in births by sourcing multiple females (up to 12) which combined with the three females from Adelaide (one elderly) would give Tomkay and Djibouti large harems of around seven females each. Upcoming males would then have females to acquire as they mature.
Taking into account the space Monarto has I would hope they would have a very very large exhibit. If Melbourne's exhibit has a capacity of 40 baboons, one at Monarto could certainly hold up to double that at the very least!
