>>>But is it possible to say Every yid is a big tzaddik?
In the literal sense that some seem to be taking it , no.
In the actual sense that it us being said that we regard every who tries to be an erlicher yid as a big Tzadik (at least in some areas) then yes it is can be regarded as a true statement
>>>Is that the meaning of the passuk “V’ameich Kulam Tzadikim”
The actual posuk in Yeshya is a part of a nevua that seems to be referring to after the coming of Mosiach. Being that it is frequently quoted in relation to todays time, it needs to be stressed that the posuk says V’ameich meaning those who are acting in a manner that makes them a part of Am Hashem.