Hi Laura,
You can use different hubs w/o problem, as soon as the reels are of the same size. However, refrain from using different size of reels (e.g. 7" and 10.5") with this machine, for critical recordings/playback as this will affect the stability/range of tape tension, hence tape-head contact.
In other words, what matters is that the internal diameter of the reel, where the tape rolls on, is the same on both sides.
Best regards,
Tod