I'm afraid that doesn't follow at all. Life requires an energy source and the only energy sources a rouge planet has, are leftover heat from its formation proc

I'm afraid that doesn't follow at all. Life requires an energy source and the only energy sources a rouge planet has, are leftover heat from its formation process and nuclear decay. The energy released by geological processes is too localised and too little to allow for widespread higher lifeforms. Even the comparatively active Earth doesn't generate enough heat on its own to keep its oceans liquid (e.g. 100mW/m² at the oceanic crust and <70mW/m² at the continental crust) or a gaseous atmosphere of any kind. In order to generate enough heat to keep a liquid ocean and an atmosphere (both are required for higher lifeforms as of our current understanding), the planemo needs to be quite large or have at least one big moon in a close orbit. The first case is very likely to generate an ice giant like Neptune or Uranus, both highly unlikely to harbour any kind of life. The second case is probably very rare - the event that led to the ejection of the planet would've also affected any moons. In addition, close orbits either rapidly decay (see Deimos on Mars) with the moon crashing onto the surface within just a few million years; or they recede over time due to the exchange of angular momentum between the two bodies (see our Moon). In conclusion, while planemos may indeed harbour microbial life in the form of extremophiles living in their crust, any multicellular life is very unlikely to have survived the ejection event and subsequent loss of a host star. Formation of life on a free floating planet is very improbable and "super advanced" life requires stable conditions over billions of years and sufficient energy, which simply isn't available for the reasons mentioned above. Feel free to correct me if I got something wrong, though.