1) No, you cannot add term together if they are not like terms, this would only work if they were being multiplied.
2) Triangles that are congruent have the exact same size and shape, all angles and side lengths are equal to each other.
Triangles that are similar have the same shape but maybe not the same size. They have side lengths that have equal ratios to each other but don't need to be the same length.
EX: Congruent: sides are 3 4 and 5 for both triangles.
Similar: Sides are 3 4 and 5 for one triangle and 3 8 and 10 for the other