First see Hoops 101
and look at Flex.
Then see the thread: Compendium of HD Offenses
Other of your questions:
Any player can play anywhere with no penalty ... although playing someone unsuited for a position will hurt (a very slow player with no BH or PASS playing at PG would not be good, for example).
Flex is designed to have a good passing "Big" and to score with shooters, not from the inside ... PASS, SPD, and PER are important team attributes ... of course, DEF and ATH are always important.
You can, of course, score from the inside if that is what your players can do. Any offense can be used to do anything in HD.