I read Tough for a couple of months back, and after I was finished I started with the prequel Koukou Tekkenden Tough. And after i was finished reading that.

I don't understand how the ending of the first story leads up to the second one.
It just feels so wrong, if you get what I am saying. It seems to me as the first series is not completly scanlated. Am I wrong?

Neither series is completely scanlated.

You can easily tell from checking how many total volumes there are, and where the last chapter released is.

Active translations list
Completed translations list
Dropped translations

That was what I was thinking, just gotta read the raws and go with the pictures 😀