Would adding new basement windows jeopardize the integrity of the foundation?
In terms of vertical load coming down, there would be little load under the first floor windows as they always have a header above the window to spread the load to jack studs on the side of the windows. In addition, there are full height king studs next to the jack studs that extend to the top plates. 

There may be cripple studs under the window to help reduce any small sagging of the window frame. Your steel angle bar may be needed to support this and the floor joists above your basement windows, depending on how much vertical dimension you want to allocate to this.

If I were doing this, I would determine the length of the floor joists and calculate the square footage supported by the joists above a window. Then use the code required load (typically 40 pounds per square foot for residences) to determine what your basement window header must support.

This link shows how to calculate load: loads-on-headers

And this link shows the beam size required for a certain load: load table