Will Agate coin be minable when main net is launched?

No, that is POW and Agate is POS.That means you will be able to stake Agate instead of mining it.

Will there be some minimum amount needed to join stake campaign?

To start masternode you will need atleast 50,000 AGT.But if you have less you will be able to join public pools.

Ha anyone from the team given the possible guesstimate on when can we expect the masternodes to be up for grabs?

Im not part of the team,but based on the roamdap i think we can expect that sometimes in Q2 2019.