Did Peter know about the reserved place and adoption at Pentecost?

No. Peter did not know about the reserved place and adoption. Peter could have said God had reserved a place and had adoption waiting, but Peter did not know that; God did. Those doctrines of grace remained unknown until God revealed them to Paul.

Les explains that in Peter's sermon to the men of Israel, Peter preached Jesus of Nazareth as approved by God through miracles, wonders, and signs, and as delivered by God's determinate counsel and foreknowledge, then crucified and slain. Les notes Peter could have also said God had reserved them a place and had adoption waiting. But Peter did not know that; God did. God's foreknowledge from eternity past knew all things. Later, after the finished work of the cross, resurrection, and ascension, God revealed to Paul all these doctrines of grace. Until Paul came, they were unknown, kept as mysteries in the mind of God.

Acts 2:23

Him, being delivered by the determinate counsel and foreknowledge of God, ye have taken, and by wicked hands have crucified and slain:

Who was Peter preaching to on the day of Pentecost?

On the day of Pentecost, Peter was preaching to Jews. He said Christ, being delivered by the determinate counsel and foreknowledge of God, was taken and by wicked hands crucified and slain. This is recorded in Acts chapter 2, verse 23.