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Small-sized beast riders can choose a pony or wolf mount at 1st level. At 4th level, a Small beast rider can also choose an allosaurus, ankylosaurus, arsinoitherium, aurochs, bison, boar, brachiosaurus, elephant, glyptodon, hippopotamus, mastodon, megaloceros, riding dog, snapping turtle (giant), triceratops, or tyrannosaurus. At 7th level, he can also choose a dinosaur (deinonychus or velociraptor).Medium beast riders can choose a camel or horse mount at 1st level. At 4th level, a Medium beast rider can also choose an allosaurus, ankylosaurus, arsinoitherium, aurochs, bison, brachiosaurus, elephant, glyptodon, hippopotamus, lion, mastodon, megaloceros, snapping turtle (giant), tiger, triceratops, or tyrannosaurus as his mount. Additional mounts might be available with GM approval.
Then you have to compare this list to Additional resources, located here.

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The feat doesn't say much because it came out later, but it should be errata'd to work with mount-giving class features if it hasn't been already. Your first question is a little confusing, but I'll take a stab at it: Yes, but it must also be an animal companion of medium size by level 7. It might also apply to naturally large companions of that level, but if so they might be overpowered if they used their own advancement progression (like +8 strength instead of 2). That's why if it also applies to them, the beast rider modifications should be used for fairness.
Ah, sniped by DesolateHarmony! Oh well.