It is okay to remove the suckers now. You can also remove in the spring.
In general butterfly bush is pruned in the early spring with the onset of new growth. You can prune back to about a foot. Also, we wanted to mention that butterfly bush is invasive. If the plant declines, you can take the opportunity to plant a native in its place. Here is our page about butterfly bush https://extension.umd.edu/hgic/topics/butterfly-bush
Thank you for your speedy, informative reply.