To say it will probably be enough, one might say 「十分でしょう」. But what if you wanted to refer to something in the past? でしょう or だろう cannot be inflected to indicate any temporal information, can it? How does one say "It was probably enough"? I thought of 「十分でしたでしょう」, which I'm prety sure is incorrect, and 「十分であったでしょう」, which I'm not too sure of.
You have already answered your question, there's no problem with putting でしょう after a past such as でした、だった、であった。
My tip for you is to google a set phrase, in quotation marks, when you're uncertain and see if it is used or not. Then you can also see examples of in what situations which expression is the better.