Welcome to the forum!
They’ll grow slowly, but keep them on the warmer end (78-82) and they’ll be a bit happier. Watch for temperature swings. They’re one of the first fish to show ich and some medications aren’t safe for clowns due to being a “scaleless fish”. Their mouths are very small so don’t feed pellets too large. I feed mine cichlid gold mini pellets and mystery snails that I’ve bred. They also take what ever shrimp gets left behind by the other fish and some of the veggies my plecos leave during the day. You can also feed brine shrimp, other types of snails, algae wafers, black worms, or blood worms.
You definitely want to upgrade though like you said. They’ll grow a bit faster in a larger tank too. You’ll have a bit more time before you have to upgrade once you get the 75 you mentioned, but sooner is better with your current stock.