Plantinga is definitely a Calvinist, though most Calvinists would call him an inconsistent one. Reformed teaching plays a very significant, if at-times tangential, role in much of his work. (For example his work on epistemology is known as Reformed Epistemology)
He's known for Molinism because of his books God, Freedom & Evil and The Nature of Necessity (among other works). Crucially his free will defense relies on a libertarian conception of freedom.
To answer your question though, I'm not sure what his precise views are. This probably lies at the heart of the difference between philosophy and theology, but philosophers are often quite difficult to pin down in terms of their own personal beliefs. Their focus is on the arguments, and they will often take up a position they don't necessarily agree with because it serves them. For example from God, Freedom & Evil:
Quite distinct from a Free Will Theodicy is what I shall call a Free Will Defense. Here the aim is not to say what God's reason is, but at most what God's reason might possibly be. We could put the difference like this. [...] He (The Free Will Theodist) tries to tell us what God's reason for permitting evil really is. The Free Will Defender, on the other hand, [...], does not claim to know or even believe that T is true.
Though given the scope of his writing on the subject it's probably fair to say that he advocates it.
In terms of how the two systems might be put together (Calvinism and Molinism), Craig seems generally optimistic that the two systems aren't all that far apart. You can find in some of his writings a variant of Molinism with a stronger view of election (Congruism) that he thinks might help cross the divide.
On the other side many (perhaps most) Calvinists are deeply suspicious of Molinism (consider this paper). For example the Westminster Confession of Faith (3.2) explicitly contradicts Molinism:
Although God knows whatsoever may or can come to pass upon all supposed conditions, yet hath He not decreed anything because He foresaw it as future, or as that which would come to pass upon such conditions.
And that probably explains Plantinga's cautious wording: he may not see much tension between the two systems, but many others do.