[johno]

I like to use either the exact same mix for potting up, or one very similar that contains mostly the original starting mix. Reason being, if the composition of the two varies drastically, water can move from one mix to the other, causing the initial root ball to either dry out or drown. So a little potting soil in the starter mix as you suggested would be okay. Just adding a little compost to the starter mix would be better.

I do like to add a little fertilizer when potting up. A little kelp meal does wonders. TomatoTone is a good choice as well. Whatever you use (if anything), try 1/4 to 1/2 strength. I also use kelp extract in the water, and add to that fish emulsion around the time of potting up - hopefully by then it's getting close to hardening-off time, and you can water with fish emulsion when the plants are outside during the day... When the plants are still young and in the vegetative phase, a little nitrogen is not a bad thing.