Can I cut the top of a small two-foot fir tree to make it fill out?

Removing the terminal/top of a conifer is seldom advisable, especially if you intend to grow the plant to mature size. Removing the terminal will cause multiple stems to develop that are more prone to breakage later in life. If the plant is spindly, it may not be getting enough sun. Firs prefer to grow in very sunny location, so if it is in the shade, consider transplanting it to a sunnier spot.