Can I keep these fish in a 240 litre aquarium?

Yes, it does fill your tank to capacity, and who am I to say if you want 35 lemon tetras, and otoncinclus couldn't bully my dead uncle. Personally I'd leave the bristlenoses out for a bit of extra room for the Otoclinclus. That's the only concession I'd make to overcrowding. Anyone who has watched the YouTube video of cardinal tetras shoaling wouldn't criticize a big tetra shoal. the y are magnificent to watch when they are all in sync.