The other day I bought a container of Hikari brand tubifex worms to feed my Bettas and as a treat for my goldfish. Niram and Kasem (both new) took to them quickly. But Boo and Rue both just looked at them and will only eat the big chunks. I'm afraid that if they eat only the chunks that it'll make them sick. Like in Jstrist's article about feeding the pellets. Right now the Bettas get a combination of ground up bloodworms (freeze-dried) and a few pellets. But only the new boys eat the tubifex worms.
Does anyone know how to get them used to the worms without eating only the chunks?

Try to pre soak the cubes so that there are no chunks. Just take some of your tank water in a small container and soak them in that, when they're all broke up dump the water into the tank. Good luck

I tried that but they won't eat the individual worms that float around.

Maybe you're just going to have to not feed those two tubifex to those guys You're right, eating big chunks can't be good for them.