If I do hyperextensions after Lunges wont there be too much of a workout for the quads and wont it lead to imbalance, harms being weak that would lead to injury?
And swim intervals do you mean by doing sprints and then a resting phase and then sprints again ? I think that is intervals... Correct me if Im wrong ;)
People do lunges all weird sorts of ways and use it to target different muscles. I think the best way to do them is to stick your lunge foot far enough in front of you so that your back knee almost touches the ground and is at a 90 degree angle, and your front knee is at a 90 degree angle. Then I explode my knee forward and up like I'm trying to jump (my back foot comes off the ground slightly.) Other people use at as a quad or hamstring exercise by just slowly going down and coming back up but I've never really understood the benefit of this over other quad or hamstring exercises, whereas I feel this is the best quad exercise.
If you do it my way you'd want to do lunges after back hyperextensions because the hyperextensions work both your lower back and glutes, and lunges just your glutes. You're not really going to imbalance yourself this way, the only concern would be that you may better spend your time by switching these two exercises every work out, but if you have the time you should do both. You should at least try the full variety of exercises that your gym/weights setup allows you.
Also, there's a lot of ways to do swimming sprint intervals. Hopefully that have one of those big clocks with just a big second hand at your pool. That way you can do something such as spring a lap starting every 50 seconds, and then lower that time or do more reps as you get better at it. Also, you can just alternate between sprinting a lap and then slowly swimming a length, like HIIT.