rob the only one that might work ( and i must stress might, it would be total trail and error and could work out expensive if it goes wrong) is parachana obscura , your buettikoferi is far to aggressive for most and will probably bully most channa , your 14" endi would eat a smaller snakehead and your leaffish would get eaten off a larger snakehead , you would also need a tropical snakehead to match the water conditions in your tank ,
pleuro would need more swimming space than 5ft and most the other channa will either grow too big or be very expensive or impossable to find . then you have the problem of tank space personally i would not house a 15" obscura with any more than 1 or two more fish in that size tank.
if it was me , i would leave the channa out of that tank , or keep the polys and a channa but loose the butti and dats , but hey man its your tank and you stock it how you wish , after all it would be poor if we all kept exactly the same thing , but whatever you decide just do not get a sub-tropical or any thing from the larger snakeheads group , this includes micropeltes ,maruila , as they are the ones you will be offered the most and are all unsuitable for your tank
oh and another problem you may face , 99% of all channa in the uk for sale are juvies around 10cm long and would be food for most of the things in your tank . a decent sized channa 13" and above will cost a fair bit in the uk expect around £70-£100 for specamin obscura a lot more for some of the other species
cheers col