This picture is helpful and illustrates the tendency for velvet to appear in between the scales: http://www.ibcbettas.org/pages/image...faq/velvet.jpg
If you are very sure that they have velvet, I would QT the fish and start the malachite green treatment with 100% water changes. If they are still very sick a week after the malachite green treatment is stopped, then you can begin to consider the antibiotic treatment. If you hurry with both then you may push them over the edge and lose them. Make sure you give them time to recover and lots of high protein food to help them along.
I would also wean them off of the salt.