SkyBlock rank is an option to get two more profile slots. Regardless, I recommend co-op slots when starting over even if you intend to play solo. I see a lot of players wishing later they had done co-op so others could help them out occassionally.
You can still play co-op alone but you can't start solo and decide later to switch it to co-op.
Yes, SkyBlock rank costs a few bucks but it is an amazing game. I figure my few bucks will support CubeCraft and possibly result in more great content from them in the future.