Thanks Bharath. Its working.Have an another query i.e. which configuration/permissions need to setup to hide all workflows(default) from start workflow dropdown. Once this step is done, i will enable custom workflows for specific user group.

Created a content workflow model by referring default workflow model 'Request for activation' and its working fine when tested. But only issue i have observed is that publisher name is not gets reflecting in 'published' column of a page in siteadmin. Its showing publisher name as 'workflow-process-s...