The Bible is filled with illustrations which God has included to help us understand doctrine. Sometimes the Bible uses the same object to illustrate slightly different aspects of doctrinal truth. The breastplate is used to provide two illustrations for living the Christian life. The answer is that there is no contradiction whatsoever, because we are dealing with two different biblical illustrations . Christ is the foundation and believers are the builders.
Some believers are wise builders and some believers are foolish builders.
The wise workmen build with materials that will endure, but the foolish workmen use materials that are consumed by fire. The fire in the illustration is not Hell, nor some literal kind of fire at the Judgment Seat of Christ. It is just part of the illustration to show that our Christian service in the Church will be evaluated. The apostles and the New Testament prophets are the early stones, and thus they form the foundation. Suppose, two Sundays in a row, a preacher likens the Christian life to driving a car.