Whats 260L...65gal or so?? I think you could get away with an RTB but your tank is pretty much stocked as it is. Your silver dollars get pretty big and you should have more than 2 of them. Clown loaches are nice but can get 11"+ and need to be kept in groups so i think it'd be a bit to much.
It's hard to say with RTB/Rainbow sharks....They usually start out docile but with age and size most aquire a temper and can become the terror's. The fact that clown loaches are somewhat shaped the same as RTB's could be a probllem...But again it all comes down to personality.
I keep mine in a 50gal with 4 Skunk loaches and afemale convict...which to my delight as worked out great. The aggressive nature of the con and skunks has really worked out great with him as he doesnt do well in any sort of peaceful community setting.