You should be using the same pen for everything. It's pretty normal for people to have small ellipse guides and work within those confines. Personally, I did a couple that were freehanded but covering bigger forms with texture was far more tedious than just working with my small ellipse guide. It's also worth noting that you are allowed to use ballpoint for this challenge, though you still have to stick to a single pen (no mixing and matching fineliner and ballpoint).