can I use an aluminium seatpost to install a tagalong on a carbon fiber frame?
It's in the realm of things where nobody can tell you it's absolutely going to be fine, but it will probably be fine. There are some wimpy carbon frames in the world. Also it depends on how much you weigh, because as far as the frame is concerned, the forces generated by the trail-a-bike are going to be added to those generated from your seated weight. If this is a road bike and you're at around the weight limit already, it's probably not a good idea.