Is a Medium-sized Paladin allowed to use a summoned Mastiff as their mount?

The Paladin spells Find Steed (Player’s Handbook pg.240) and Find Greater Steed (Xanathar’s Guide to Everything pg.156) list the following creatures that are valid targets to be summoned as mounts:

Find Steed

  • a warhorse
  • a pony
  • a camel
  • an elk
  • or a mastiff

Find Greater Steed

  • a griffon
  • a pegasus
  • a peryton
  • a dire wolf
  • a rhinoceros
  • or a saber-toothed tiger

However, of these options, the Pony, Mastiff, and Peryton creatures are classified as "Medium" creatures. And per the Mounted Combat rules found in the Player’s Handbook (pg.198), a valid mount must be a size bigger than its rider:

A willing creature that is at least one size larger than you and that has an appropriate anatomy can serve as a mount, using the following rules.

The description for Find Steed stipulates that the summoned creature serves as a mount for the creature that summons them:

Your steed serves you as a mount, both in combat and out, and you have an instinctive bond with it that allows you to fight as a seamless unit.

Is this a case of Specific Beats General, where a medium-sized creature very well could ride on these magically-conjured medium steeds, or is such a creature still bound to obey the restrictions placed on them by the Mounted Combat rules?

A related question about Perytons being summoned by Find Greater Steed touches on this issue, but is scoped around questioning whether the statistics of a summoned Peryton are different from a normal Peryton. As a result, I don’t think it’s a duplicate of this question: What happens if I summon a Peryton with Find Greater Steed?