they can be mixed, both snakeheads are dwarf species so it should be ok. I've successfully done it before. I introduced the bleheri's in the tank first then after awhile i put in the gachuas. When i first put in the gachua's there was some aggression issues, nothing too serious just some flaring and what not so i took them out rearranged the tank completely then put them back in together.
older fish are gaxhua , in firsdt 2 hours is ok , they just go watching bleheri
aver 3 hours , one of my bleheri are seriously bited on stomac , other is only on anal fin .
now , gachua is in my feeders tank , for moment.
my gachua go out of the feeders tank today , when i recame home after job i found my gachua on floor , totaly dryed like a stik , but when i take it he moov is pectoral fin .
i put in tank whit nose out of water ... 1h after is alive !! héhé
yeah make sure you have a secure lid on your tank. Snakeheads are known for jumping out. I've lost countless snakeheads because they've jumped out. Some have survived for a couple hours then died, some survive. Good luck with yours keep us updated.