I found the FishBase page: http://www.fishbase.us/summary/Betta-midas.html.
I would keep them like most other Betta spp. from Borneo. This means soft and acidic water. PH around 5 should be fine. Breeding shouldn’t be hard when fed plenty of live food like white mosquito larvae.
They are ‘big yellow’ Bettas (waseri group’ from Kupuuas – I’ve found them alongside B.mandor, P.ornaticauda and P.anjunganensis. Although habitat is as stated, they are tolerant of most water conditions and easy too feed although they can be a bit boisterous !
Betta midas isn’t a member of the waseri-group altough they look very similar. B. midas belongs to the anabantoides-group.
If you found them there we now also know where they live.
With this info you should be able to give them a good home, I guess.