Can we have a single authentication scheme in CA Siteminder where user would be tried to authenticate using IWA and if it fails then automatically it would go to Form Login
Not easily, but it should be possible. Since IWA is handled by IIS and not SiteMinder to some extent, you need to customize the IIS error messages. So, on an IIS error, send a redirect to a resource protected by forms authentication. I've never tried it, but it should work.