I'm trying (and struggling) to make a sub-category similar to Portal's "Least Portals" category extension. Does anyone know how to make something like this so the leaderboard is ranked based on amount of portals and time?

As far as I can tell, Portal's Least Portals category is still sorted by time taken but if it's discovered that it's possible to use fewer portals to complete the level (and that with fewer portals you cannot match the existing times) then current runs are invalidated. One case where it appears a method was found to use fewer portals also has one run with more portals but it's much slower. I don't know if it's possible to implement a combination of the two (time and variable) which simultaneously change leaderboard standings without making separate subcategories, but you can do the same version of theirs with variable annotations and make it so a faster time obsoletes other runs (or doesn't). Although it should be possible to manage such submissions manually, unless it's an incredibly popular leaderboard.