I think so, Iam interested to know, so I hope you don't mind me latching on to this question.

When you say seedlings how far along are they?

well they are stocks and asters nearly ready to go into pots with a good few leaves on some.

I wouldn't use any extra fertilizer on any seedlings - there will be quite enough in the compost. It's like trying to feed a baby steak and chips! :-)

Once the plants have grown several pairs of leaves and have a good root system you'll be potting them on or up into larger pots, so you'll be adding fresh compost and so they won't need any extra for a while.
You could start using a soluble feed when they've been in the pots for about a month, or mix some slow release granules into the compost when putting them into their final containers.

I agree at this stage I just repot them

Ok Thanks everyone :)