Just to clarify, if you get any other fish, you will need to take the swords and kuhlis back.
The swords grow very large! I have some that are maybe 6 months old and are 4" long not including the males sword. The ones you see at pet stores have sexed out but aren't close to full grown yet.
If I were you I'd look into a trio or foursome of guppies or endlers and some shrimp. Sounds like you are looking for the tank to have some activity and that would provide it, but you'd want to upgrade with them eventually too if you want them to breed. Guppies might be a better choice since they are more likely to eat their fry, but endlers can work if your brother wouldn't mind getting rid of the fry eventually