Yes,Shaymin is a legendary pokemon like Mew,Celebi and Jirachi
Yes it is and the fact that there are many doesn't mean a single thing about if they are legendary or not! Go check Latias Emerald pokedex description:
"They make a small herd of only SEVERAL MEMBRES. They rarely make contact with people or other POKéMON. They disappear if they sense enemies."