About legendary birds, yes, they are unavailable but they might be in the future, if they implement dungeons, which will take a while.
The only available legendaries are Mewtwo and Rayquaza, but you can only keep them for a short period of time, not permanently.
Squirtle do appear on surfing but i'm not sure if it's possible to encounter them through fishing. Remember that squirtle appears only during day time, this also applies to all other starters as well.
You'll have to keep trying, they are pretty rare and are around 40 level if i remember correctly.