Housing snakes of different species together is just flat-out stupid. If you don't have the room/money for another enclosure, don't get another snake. I would advise your friend to separate their pythons and boa immediately. Different species have different environmental parameters that need to be met. Also, housing animals together allows them to stress each other out, which leads to a whole slew of health problems. If you have your animals separated, you can prevent future disease/parasite outbreaks from spreading. And finally, boas and pythons can both get a disease (I forget what it's called) that shows little to no symptoms and, while survivable for boas, is 100% fatal in pythons.

They may "get along" (it's probably more just tolerance than actually enjoying company), but keeping them together for extended periods of time is really a bad idea. If you have them out for an hour or two and let them interact it's fine, but not 24/7.

Now that that's been said, smaller carpet pythons (I believe the smallest is Irian Jaya), blood pythons (short but HEAVY), dwarf locality boas, rainbow boas, and Dumeril's boas usually stay 5 - 7 ft, and if you're looking for even smaller, spotted and children's pythons don't usually pass 3 or 4 ft.