A CR 1 creature is comparable to a level 3-4 PC by itself, but as an animal companion using that power is pretty limited. For the beast to attack, the ranger has to not attack, which means overall as long as the companion isn’t much more powerful than the ranger’s attack I don’t see the issue.
Take a Copper Dragon wyrmling compared to a flying snake. +2 AC, a lot more HP but they will match by level 6, and you can start it lower than average. Damage of the main attack is about the same. The breath weapon is powerful, and if you leave it as unlimited use (has to recharge, but will get it at least once every battle) I think it could be an issue, but limited to say 3 times per long rest it would be no more powerful than a magic item like wand of fireballs.
It has a few more skills and resistance but don’t think those would be a big deal. It can talk which is a huge help but that rangers can’t talk to their beasts is pretty lame.
A Halfling Ranger could ride it as a mount which would be very powerful but for a dragonborn it is not an issue.
The creature being intelligent is different from a normal companion but not really a issue; intelligent, loyal animals are a pretty common fantasy trope. One problem if you plan a lot of urban encounters may be most inns aren’t going to let you keep the dragon in your room. Is dealing with that going to be fun for the party or a distraction?
Might want to discuss it with the group to see if they like the idea or they think it is a problem.