RG's are notorious jumpers ... so are 6 line wrasses and firefish ... clowns are known to jump accasionally ... all these fish meet the size requirement so the only way to safely keep them is with maybe a eggcrate lid ( that way you dont have a temp buildup as you would with a glass top ) ... also maybe have a muffin fan blow across the top of the water to keep the temps down
Going bigger solves alot of problems as far as stability ( take it from someone that has kept very large reeftanks
) ... so the bigger you go the better