That cage will last longer for an MBK...I've kept white oak grey rat snakes, they are great pets but get about 5-6' and do like to climb, just like the rest of their 'family'
of similar U.S. rat snakes. They are both great choices as pets to handle, & easy to feed on f/t. I don't think BPs care about (feel threatened by) the presence of a king
snake, since kings are not native predators where BPs originate (Africa), but I cannot promise that a king won't be a little "inspired" by another snake of any kind nearby,
though when you raise them on f/t rodents I really doubt you'll have any issue. I had a house-full of snakes when I had MBKs, with various kinds of snakes housed nearby & no issues whatsoever. MBKs are usually voracious eaters, so if you forget to wash your hands well your king snake may "remind" you, lol.
A hatchling grey rat snake will be ok for a year or so in the that Exo-terra but will need a bigger cage once it grows more in time: I recommend using a large glass tank (50+ gal. is nice) with a screen top...they do best with air flow IMO, humidity doesn't have to be exact or like where they're from...I kept them when I lived in the desert without any issue, & they're calm pets. I would say they're smarter than kings including MBKs, when it comes to not confusing your hands with "food", and a king snake (of any kind) may also notice the scent of another snake you've handled on your clothing, so just washing your hands may not dissuade a king snake, depending on the individual personality.
A grey rat snake will use UTH at one end of cage, & overhead basking light or CHE in winter (supplemental heating) once they grow bigger (& braver) about spending time on branches. Good luck deciding, both MBK & a white oak grey rat snake are very attractive pets, & both are easy straightforward to keep w/ cool & warm side hides.