After passing quarantine a juvenile common BCI is fine in the same rack with your adult ball pythons. Also if you get one of the smaller/dwarf locality boas it will never outgrow that rack - my Tarahumara females adults are fine in 41-qt tubs, and the male adult is in a 32-qt.
If you get a true BCC red tail boa, such as a Guyana or Suriname, they could eventually outgrow a 4x2.
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