Imbellis are the same as splendens just not extensively bred for fighting but are still aggressive.
In such a set up you'd need a ton of caves, plants surrounding caves, tons of java moss (great hiding place ;P). Even more caves, only a few males and a lot more females. I would only attempt this with sibling fish as in you breed your fish, put them in the huge tank and let them stay there. Eventually though.......they will breed. Meaning a lot of fry will get eaten but some will survive.
Keep in mind every male will need at least the area of a 2.5 gallon tank as a territory, thus the need for a large tank or pond....as big as possible, preferably over 1000 gallons for a pond.
I still wouldn't attempt this. Like Adastra said what would happen in nature dosen't apply to these fish.
Why bettas fight: They're control freaks...like major. They will fight and eventually create the alpha dominance issue....remove the alpha fish....you get a blood bath.
Leaving the male with his fry: Experminets have shown that a male betta will remain peaceful with his fry because he's the alpha fish and the fry grow up knowing this....a possiblity for this set up.