The clowns should be happy in there.
Yes you can have a nem in there if you have sufficient lighting and clean water, but i would personally hold off for awhile if the tank isn't mature.
Since you have a sand bed you could keep something that likes its foot buried in the sand like a long tenticle, or you could do something like a Bubble tip that like to be up on the rocks. Just keep feeding to a minimum so the nem doesn't quickly outgrow the tank.
Get one large enough that it can get bullied a bit. Some clowns can be rough on them.
I'm not advocating it, but my very large female YSM clown lived in a 10g for 3 years at her full grown size before she was re homed with me. She was fat, healthy, and a real menace when i got her. She is now queen of a 300g stock tank. There is no question she is in charge even in the 300g. Obviously a 10g is not ideal, but they should be fine.
Good Luck.