Probably about another inch or so, but by that time your CK would have grown too. Is it possible to try a devider and see how they react to eachother first? My RCK is the boss of the tank and doesnt tollerate small fish very well. If he cant eat it, he will terrorize them for life lol.
Oh well if he hasnt eaten that little guy yet, you might be ok. Just keep the ck well fed and keep plenty of hiding spots for the little guys. If worst come to worst - convicts dont cost too much to replace lol. Good luck man. Keep us posted on this one.
Upgrade to a bigger tank or get rid of the Clown Knife, they can grow up to 28" and yes it'll eat your convict, especially if you've fed it live food in the past.