You can use "했어야 했어요" in any situation. For example, 나는 운동을 했어야 했어요. 당신은 운동을 했어야 했어요. 그는 운동을 했어야 했어요. 그녀는 운동을 했어야 했어요. 우리는 운동을 했어야 했어요. 너희는 운동을 했어야 했어요. 그들은 운동을 했어야 했어요. All makes perfectly sense. Also, in case of "I" or "we" (in other word, including "I" in the meaning), you can use "할 걸 그랬어요". Similarly, in case of "you"(너희) (including "you" in the meaning), you can use "하지 그랬어요". For instance, 나 좀 일찍 출발할 걸 그랬다. (나 = I) 우리 좀 일찍 출발할 걸 그랬다. (우리 = we) 너 좀 일찍 출발하지 그랬어. (너 = you) 너희 좀 일찍 출발하지 그랬어. (너희 = You)

Korean does not change depending on person or number but by hierarchical construct. Now I did mention it doesn't change by subject's person/number, but it can change meanings depending on that OR be wrong. As for the case for ㄹ걸 (그랬다), I've seen it used for 1st person (singular or plural) mostly, but 2nd person also... not 3rd though... -지 그랬다 is mostly sensed as though you are saying it to someone. (2nd person) Hope this helps.

For subject "I" "할 걸 그랬어요" For subject "you" "하지 그랬어요" How about for subject he/She? How about for We/You (너희)/They?

I didn't get what you asking. Can you explain more?