Hi Mahesh,
For your question 1, since you are using Exchange 2010 SP3 and only migrate 150 users to Office 365, you can deploy cutover migration.
For your question 2, if you deploy cutover migration and SSO, you need to hide all the company A users in global address list, and then perform a cutover migration. After the cutover migration is finished, you can implement the AD FS to enable SSO. For more detailed information, please visit this link: http://community.office365.com/en-us/wikis/exchange/835.aspx
For your question 3, when migrating the accounts with cutover migration, in general, the users can access their emails through Outlook without any impact. You can get more detailed information about "cutover migration" from this link: http://help.outlook.com/en-us/140/ff628719.aspx
Thanks,
Edward Qu