If you click "edit this person" on a profile page and go to relationships you can change a relationship from biological to adoptive or step or a number of other choices. You can add more than one marriage fact. Neither of those would be possible if it wasn't appropriate.
Some adopted children may have no desire to discover their biological family tree. That's their choice.
Are you not going to research a first or second family because they're only your 1/2 cousins? Of course you don't HAVE to include them. It's up to you but it certainly is appropriate and correct.
Barking Up the Wrong Tree Blog