Folks will typically get 6 foot cages for their boas to live in the rest of their lives, though the general rule of thumb is that the MINIMUM length of the cage should be no less than half the length of the snake, ie an 8 foot snake in a 4 foot cage (doesn't mean this is the best avenue, of course, as minimums are never ideal). Boas WILL utilize vertical height if it's given to them, but temps get a little trickier to maintain.
For your own personal comfort level, getting a baby snake and growing with it is a great way to get used to the size of the boa. Kinda like having a kid... they'll always be your baby, even if they appear intimidating and huge to other people lol
Boas are the best (yes, I'm biased).
EDIT: I knew I missed one of your questions! As for other pets, I've had large boas in the same house as my cat, and never had an issue. It's all about maintaining awareness. Currently, the litter box for our cat is in the same room as the reptiles and has been for two years, and no one's gone off feed in two years except for a couple winter months. When we need to clean cages or get the snakes or lizards out for any reason, the cat gets locked in our bedroom.
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