Imbellis is believed to be the same species as splendens, try smaragdina.
All are aggressive, splendens being the most aggressive.
I agree with what Adastra said, start with imbellis or smaragdina and breed them to dragon splendens...those would some amazing fish!! Even though these crosses have been made to create dragons in the first place lol.
IDK what finnage you'd be able to do, long fins are less aggressive but can damamge the fins more easily. HMPKs might be a good start.
Who am I? Well I am a Junior Member of the International Betta Congress and a breeder of white HMPKs, red dragon HMPKs, black/red dragon HMPKs, metallic multi HMPKs, blue masked HMPKs, and now multi color HMs. Soon to add black HMs. I will be showing in New Breeder class in fall of 2012.