Can you use the term isoclels to describe a rectangle?

Answer 1

No, apart from the fact that there is no such word. The word isosceles is derived from "equal legs" and, in the context of a triangle refers to the two sides or legs of an isosceles triangle in its conventional aspect. In the case of quadrilaterals, any of the following have equal legs: a kite, arrowhead, an isosceles trapezium, a rhombus, square, rectangle, and at a stretch a parallelogram. All in all, then the adjective would not be particularly helpful in narrowing down the possibilities.