Assuming 240 litre aquarium is some variation of 100 - 120cm long x 40 - 50cm wide, a group of Bolivian rams would be fine
Adding in 4-6 additional fish would be less likely to cause Chester distress (I suspect he/she would be quite pleased with the company) than attempting to add a single Bolivian ram - in which scenario, one fish dominating the other is rather more likely
If you search out juveniles, Chester will likely be Boss Fish
As you’ve had Chester for 10months, I’d guess female but adding a known male is no guarantee for a successful pair, like most dwarf cichlids, they prefer some selection