Hi,
Personally i wouldn't add them at all unless you plant to upgrade again in future, clowns grow 12" and its surprising how fast they grow, mine went from 1.5" to 6" in 2 years and i keep them in a 400 litre tank.
There are loads of other lovely loaches which could live out their whole lives in your tank, zebras, darios, chain loaches to name a few. Have a look on www.loaches.com for more.
Emma