1) since they block out the warp and such, could they disrupt the effect of synapse in tyranids? could they block out/negate some of the abilities tyrants get? (like warp field and such)
I see no reason why not.
2) the geller fields in ship block out the warp so it doesnt affect the ship. could a ship without a geller field be fine in the warp as well if all its crew were pariahs? (now i know there would never be a ship composed entirely of pariahs/blanks. it's just hypothetical)
Hmm.. Well we know that blanks have a negative effect on Warp entities/Psykers in the Material Plane, but what effect they would have upon Daemons (as an example) in the Warp itself is up for debate.
3) if a blank were to walk up to the Emperor in his current state would it kill him? (again hypothetical)
Well the Sisters of Silence were Blanks right? They fought alongside the Emperor during the Great Crusade. They also fought in the Webway against the hordes of Daemons that threatened to over-run the Imperial Palace - and lost. This leads me to suggest that there 'Blank' effect is not all that powerful against Daemons.
As for what effect they would have on the Emperor in his 'current' state? I imagine that the Emperor is too powerful a Psyker to be effected by Blanks.
4) are there any space marines that have blanks/pariahs in their ranks knowingly or not?
Not that I am aware of. Blanks have an effect on non-Psykers aswell, in fact they have an effect on everyone/everything with a Warp Presence, If there were any among the Astartes Recruiting Worlds they would probably be killed or recruited by the various Imperial Organisations that makes use of them.