Is, “my pen roll has more space” a valid justification for buying more pens?

it will probably work on a dip pen as most nib holders will happily take nibs in a range of diameters. But not a fountain pen— it must match the diameter and shape of the feed and section. So no, a particular nib won’t work on any fountain pen, it will only work on the pen it was designed for and ones that are similar enough to it.