Good fish for an Asian Biotope?

if you want a community tank, stay away from blue gouramis and the like, and tiger barbs.

a group of 5 pearl gouramis will look great in that tank

It depends on how strict a biotope you want to do. The true biotope tank attempts to replicate the EXACT location where the fish could be found. That means not just fish from asia or southwest asia, but from a blackwater stream on this particular side of this particular country leading into this particular river. Biotopes need to take into consideration not just the country of origin, but the type of water and the type of fish that would be found together, taking into account that some fish occur together because one is food.

So find out where red-tailed sharks would be found in Asia and then find out what fish would be with them. You can add plants, rocks, leaves, and wood according to the biotope needs.

I looked it up, the red tailed black shark would be located in: Clear and muddy rivers with wood and stones. Southeast Asia in the Me Nam Chao Playa basin to the Paknampo region in Central Thialand.

you could have:
(the pearl gouramis wont do well with a red tailed black shark, i made a stocking list for both situations)

4 pearl gouramis
a school of 10 harlequin rasboras
a group of three or four yo-yo loaches

1 redtailed black shark
6 zebra dainios
a school of 10 bigger rasboras