Adjudicating "Specific Beats General"
The wording in Find Steed says the creature "serves you as a mount", without qualifiers for size, but with qualifiers for creature type. So answering this question is contingent on whether we accept that this line is more specific than the nominal rules for Mounted Combat.
If we accept that the Mounted Combat rules stipulating a creature "at least one size larger than you" are more specific, then the answer is a flat NO: a medium-sized creature cannot ride any of these three creatures as mounts; although nothing in the rules precludes them from summoning any of these three creatures.
If, however, we accept that the Mounted Combat rules' stipulation on creature size is less specific than Find Steed's blanket allowance of any of its specific creatures to be used as mounts is more specific, then the answer is almost YES: a medium-sized creature may ride these creatures... But with one important limit.
A Medium-sized Paladin may ride a Mastiff or Pony, but NOT a Peryton
The reason is a subtle difference in the wording of Find Steed and Find Greater Steed.
In Find Steed, the spell description is very explicit that the summoned creature serves as the character's mount:
Find Steed
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. While mounted on your steed, you can make any spell you cast that targets only you also target your steed.
However, in Find Greater Steed, this language reads differently:
Find Greater Steed
You control the mount in combat. While the mount is within 1 mile of you, you can communicate with it telepathically. While mounted on it, you can make any spell you cast that targets only you also target the mount.
"Controlling" the mount in combat is a different claim than "serving as a mount". One simply notes a character's control over the mount, the other implies its specific use as a mount. So if we accept that "serves you as a mount" is indeed a "Specific beats General" interpretation of the Mounted Combat rules, then a Mastiff or Pony (summoned by Find Steed) are perfectly valid, but the Peryton (summoned by Find Greater Steed) would not be.
It probably depends on the style of Campaign being run
Ultimately, the question wants to permit (in the most extreme scenario) a 7-foot-tall Half-Orc Paladin to ride a 4-foot-tall Mastiff as a mount, and while there is a compelling argument to make that the rules-as-written permit this kind of interaction, interactions like this could interfere with the narrative consistency of your campaign.
Also consider the weight restrictions of a mount. A Mastiff has a maximum encumbrance of 195 lbs. This is enough to carry most medium sized creatures only as long as they are not wearing any armor or carrying any equipment.
On the other hand, if your table plays more fast-and-loose with the rules, or has a generally goofier atmosphere, or generally prefers that mechanics supercede narrative logic, then go ahead and allow it. In terms of balancing concerns, the 3 creatures mentioned aren't markedly more powerful than their peers; in the case of the Mastiff, the creature is only a CR1/8, and considerably weaker than other possible mount options offered at that level, so allowing a Medium creature to ride a Mastiff into combat is not likely to offer any meaningful combat advantage over simply fighting alongside it, or (pertinently) using a much stronger mount like a Warhorse that also obeys the size restrictions set by the Mounted Combat rules.
The Peryton is worth mentioning though, in that with its Flyby feature and nominally high-powered damage capabilities, it is considerably more powerful in combat than its peers, and may be highly competitive. So from a purely mechanical perspective, having it have an obvious drawback (cannot be mounted by medium creatures) may be a necessary balancing mechanic.
