If it is not possible to stud mount the accelorometer on non magnetic pipelines what is the best method recommended to take line vibration? If we use six inches probe , will it reduce the natural frequency of accelorometer drastically enough to leave the readings doubtful?
Secondly, while taking diplacement spectrum, I have found the analyser showing overall p-p readings less than what the spectrum reveals on certain frequencies? How is it possible . In case of velocity, overall is always bigger than velocity at any particular frequency as I have found it to be?

Use an epoxy glue. Pick that carefully as some epoxies have a rubber compound in the formula and can create another issue.

Some time ago, we used straps (big hose clamps) for this for certain applications.