Absolutely! Don't put them in the same line if you can possibly do this...make another row, offset from the first at a minimum of 2 feet, 3 feet would be better. You should have equilateral triangles between 2 in the back and one in the front row, the new row.
Making a row offset in front of your original row will give reason to the height difference as well as plugging up the gaps. This gives each plant plenty of room to grow symmetrically and fully. Your original shrubs should be topped to the height you are easily able to maintain. That will help them to grow outwards versus upwards. Become denser. Just brush the tips on the sides and make sure the bottom of the shrub is wider than the top.
Have you fertilized? A tree stake of balanced fertilizer works great for a year. If you've already fertilized how long ago and with what formulation? Compost is not fertilizer. Does have some nitrogen but you need a balanced fertilizer. Are your new shrubs in pots or B and B?
A picture of a similar privacy screen. Neighbor's garbage is behind this screen. Solid fences make you feel like you are in a cat box. These screens with the 2X2's lap jointed into regular squares are also amenable for a great vine. This would be an alternative to the staggered rows...Use hops, or Akebia quinata? Need to know where you live for better suggestions.