would be kinda funny to see a lima eat a rtc, but if there is a vast size difference in how much larger the lima is than the rtc then yes he might eat him. If the rtc is wider than the lima no worries, but worry for the lima in a few more months.
Another note. Couldn't help but read that he's going into an outdoor pond in Iceland? Is it fed by a geothermal vent? If not I doubt he'll survive the climate there very long.