1. To accomplish turning a
River Drake into a mount, there are a few obstacles to overcome. Many of these obstacles could be accomplished through roleplay, as apposed to mechanical class abilities or skill checks.
First and most simple, the River Drake is a medium sized creature. If your Ranger is a medium sized character, riding will not be possible unless the Ranger is Reduced, or the Dragon is Enlarged. If your Ranger is a small sized race, pass this obstacle and consider the others.
Secondly, you have the INT of 8 and the description of their character/nature. Lines in the creature description such as, "Preying upon fish and fishermen with equal ease", "river drakes are cruel hunters", "...such a bribe is unlikely to stave off a particularly hungry individual" all suggest that the idea of a River Drake doing anything other than eating a humanoid is unlikely. Even the suggestion of treasure bribe only serves to momentarily distract the Drake, not influence it positively. Also, the Drake's natural tendencies are to retreat strategically and prefer it's lair near water. It appears territorial. Leaving such comfort, it's storehouse of "aged meat" and treasure is not something this creature would be likely to do.
To directly answer your question, what you would need with the above creature, as described, would be a very high level Diplomacy check. Likely repeated Diplomacy checks with an ever increasing DC as the Drake continues to be convinced to resist it's natural values. Since it seems to value eating/hunting/collecting, perhaps the promise of ongoing, successful food opportunities and treasure could be used as a roleplaying enticement. The Ranger would need to make good on such an offer and accept that the Drake would be cruel in it's hunting methods. I don't know how likely this will go over with the Ranger or the rest of the party.
Thirdly, if the Ranger possesses an Animal Companion, I'd imagine inevitable conflict that would result in the Ranger quickly ending up with one or the other. Cruel intelligent hunter vs. loyal animal is likely to result in distrust/dislike on both sides. I'd expect a fight, likely to death or fleeing of one side.
All this sounds unlikely to be possible, however it could add some fun roleplaying tension for a while. Ultimately, even if he did convince it to stick around for a short time, I don't see such a creature cooperating for the long term.
2. While I can't recall reading anything that would say ALL true dragons would kill lesser dragons on sight, since the different dragon types vary widely on their basic character and inherent personalities,
Blue Dragons specifically aren't known for warm, fuzzy feelings. They're character make them seem more likely to scheme to make minions than instantly attempt to slay another creature on sight.
You're the DM, you get the final say on all decisions on the nature and responses of these creatures.
