A chilwa of between 3" to 6" works very well for mahseer, mulley, silund and some of the other catfish.
I use a single 6/0 hook mounted just below the dorsal fin (have we been down this road before)
There are many other interesting ways of mounting a chilwa that need to be seen done live.
I disagree that crabs are used only in rapids, as crabs are very light and get swept away in the current, making it very difficult to place your bait in the right spot.
You may need a lot of lead weight in such cases which is not always a good thing when fishing snaggy fast water.
I usually use crabs in deep pools,or places where the rapids even out.
Chilwa always needs lead, and i would say more than what is used for ragi, as ragi in itself is pretty heavy stuff.
Chilwa tends to float on the surface and attract more birds than fish if it is not weighed down properly.
The idea is to bump the chilwa down the rapids, so that a waiting mahseer can have a go.
I would say that chilwa is more productive in rapid water compared to slack water, where it takes on a rather dead appearance.