Is it true that if a governmental employer who has opted out of social security and has now decided to provide a retirement plan, could adversely hurt the social security of an employee that worked in the private sector and paid into SS for 10+ years?

Does it matter what type of plan is started? SEP, MPP, 401a or DB?

Don't know about a SEP, but if any 401a plan, yes.

A good write-up about this may be found at http://www.reish.com/publications/article_...m?ARTICLEID=682

Thank You... have you ever heard of participant being able to OPT IN to social security even if the employer does not?

I've heard of ministers who conscientiously object to participating in Social Security being able to opt out on an individual basis, I think using Form 4361. But never heard of employees of a local government that opted out of Social Security being able to opt-in.