No. Similar figures need equal sets of angles. The angles of a square, for example, need not be equal to those of another rhombus.
No, not all the equilateral quadrilaterals are similar, it is not necessary. You may make any equilateral quadrilateral taking any measure.
Not all equilateral quadrilaterals are similar because while they may have equal side lengths, their angles can differ. Similarity requires both equal angles and proportional sides. For example, a square and a rhombus are both equilateral but not similar due to different angles.
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