If the marble doesnt eat the rtc while its small the rtc will eat the marble when its big. If the marble eats the rtc it will likely die as well. So I would suggest separating them or possibly losing both in the short-term or the marble in the long term. So, to summarize, you will lose at least one fish whether its now or in the future.
i would move the achara like he said it would get eaten in the long run by either the hybrid or the rtc. but the hybrid and the rtc should be ok together long term.