Peace be upon you. It is permissible to follow Sunni Muslims in prayer for the sake of maintaining Islamic unity, and the prayer with them is valid and sufficient. (Source: Ayatollah Khamenei, Answers to Questions, S600). Additionally, it is permissible to pray behind them, but the follower should recite quietly for themselves if possible. There is no harm in intending to follow their imam, but without following the rules of congregational prayer. (Source: Ayatollah Sistani, Hajj Rituals and Related Matters, Rules of Congregational Prayer). May you be successful in all good.