One Tween is enough. Tweens can interpolate a lot of properties in the same time, just like AnimationPlayer. First You add lines of code to interpolate property, but they begin to interpolate only after You command tween to start :
Tween.interpolate_property("speed",0,5....and so on)
Tween.interpolate_property("size",5,10...and so on )
Tween.start()
Tween emits signal "finished" whenever any property is done interpolating, and "all_finished" when last of them is done interpolating. Check exact signals names in signal tab or in tween documentation