There is a rather amusing picture of a redtail cat inhaling a clown knife floating around on the net somewhere. RTCs will eat anything that they can catch/swallow...So hope the knife grows faster than the cat lol.
I don't know...I'm going to disagree and say it should be fine, but only for awhile since the RTC will get bigger than the CK. But of course...it also depends on the size of the tank you're going to keep these beasties in.