A 3 gal pico is pretty tight for a flagfin, as these are swimming species. However, I know that bluestripes often come in very small, but I digress...
Rics may or may not be OK with a small flagfin, depending on the pipe's habits (whether it perches on coral) as a ric could definitely swallow a bluestripe. I'd be concerned about the constant proximity between fish and coral.