Hi Padmaja. It's too bad you didn't catch the mealy before plants got so infested.
A cotton ball dipped in Rubbing Alcohol kills mealy, but since your plants have so many bugs, it'd take a lot of cotton balls and rubbing alcohol to do the job.
Any plants without mealy should be moved from those that do. Mealy are very contageous.

If they were my plants, (if it's possible) take outdoors, remove plants from pots..discard old soil, scrub pots thoroughly. Hose roots, leaves and branches, using dish soap and warm water.
Pot up in fresh soil, to cleaned pots. Keep plants outdoors (weather permitting) in a semi-shady spot.
Since insecticidal soap killed some, perhaps a second or even third application may be needed. Read instructions on container for application and dossages.
I spray with Fish Emulsion in fall to keep bugs, mainly scale, at bay. It works incredibly well, but don't know if it'd kill mealy.
There are mealy insecticides on the market, which may or may not work, but if cleaning doesn't work, I'd look into it. Toni

Marathon 1% systemic insecticide granules are good for mealybugs, whiteflies, aphids and all kinds of things that suck sap on ornamental plants.

I get them on my mandevilleas sometimes, the best thing that works for me is NEEM. Alcohol works okay too but the NEEM seems to work a little better. If it is in a pot you can just tip over the whole pot and get to the stems under the leaves better. I usually cut back my MOO jasmines in the early spring and treat the stems with NEEM too.