can ferrite rods be combined in series or parallels giving the same effect as a solid ferrite?
Yes, you can combine any shapes but there are also different classes of ferrites so combining different ones will give common properties. If you use identical ones, you can connect them with glue or even adhesive tape. It's important that they stay close to each other. There are also slots in the cores specially.
thank you
I was able to secure 4 identical ferrite rods from the 1980's era from Ukraine for my coil experiments.
if you want to connect two ferrites together and they are flat then the best is superglue if you use high frequencies. Low frequencies cause vibrations and a special soft adhesive is required.