No, Israel would use full size nuclear arms against Iran. And that would be due to an imminent or launched attack on Israel itself.
It's called deterance.
"But would Israel use tactical nuclear weapons - if it had them - to do so?"
No they wouldn't. Because if they did that would open up Pakistan and India using them in Kashmir, South & North Korea using them. It would legitimatize the use of nukes as a preemptive measure, which means NK may get one off against the US and with a RAT congress in, we may not get to retaliate.
What Israel should do is threaten to launch a conventional attack on Irans oil terminals. The reason Iran is not getting touched is because they are hiding behind their oil. Threaten to destroy that and now see how quick China and Europe tell Iran to stop.
Yes. I am remembering certain Iraqi nuclear reactors that were under construction several decades ago.
This will only go so far...