original xcom behavior: if you are in targeting mode and click on the "next soldier" or "previous soldier" button, the targeting mode is terminated and you switch to the next soldier
openxcom behavior: if you are in targeting mode and try to switch soldiers (clicking on "next soldier" or "previous soldier" OR <TAB> or <SHIFT>), nothing at all happens
I don't know which makes more sense, honestly.
about "TABing" through enemy units... you can center on an enemy in view by pressing the number key corresponding to the little red square. as for shooting at enemies outside of your viewing range, well, tough luck... you'll just have to hunt for them in the dark. I can't think of a decent way of implementing what you want. unless perhaps using some kind of key combination for "switching to another soldier without changing the camera", so you can keep the camera centered on the alien. but then you'll have a hard time finding the correct soldier to shoot with.
if you have a better idea, go ahead... I don't think something like "cycling through all enemies seen by any soldier of your team" is truly helpful.