@dawn16 There are many ways to improve the stability of the shoulder blades, the one you linked looks good.
Rolling forward is common in dips with bad form,
here is a demonstration how it looks like. Having a good posture overall does not mean you can't lose it during an exercise if you haven't learned good form.
I don't do lat pulldowns since I train at home. During pullups I start in a passive hang, externally rotate the shoulders (so the elbows face more inside than back, if you use the overhand grip it's like trying to twist the bar into an underhand grip), unshrug them from your ears (scapular depression) and initiate the pull.
The exercises that I do for scapular health are horizontal pulling (row variations and front lever work), dumbbell external rotations, stick dislocates and the chin up dead hang to stretch the lats. It works for me but it's just an example.
In general your shoulders will be healthy if your traps, rotator cuff and serratus anterior are strong enough and the internal rotators like lats and chest are not too tight.
By reading resources like the one you linked you will learn more and you can choose your own exercises.