why do they need more soil?
Thats ok, but how long are they getting? if they stretching too much you may need more light.
i've heard those pellets are good for something i forget what i'll have to look it up. it shouldn't hurt the seedlings.
http://www.planetnatural.com/site/alfalfa-pellets.html
yup found it. guess they have a NPK ratio of (5-1-2)