First off, all seeds contain enough food for a young plant. You don't
want to be "feeding" them at this stage!
Personally, I would wait until after the tomatoes develop true leaves
- not the cotydelon (sp?) leaves. You'll know true leaves, because
they are what look like tomato leaves. It's kinda like baby teeth,
these primary leaves will eventually drop off. When the true leaves
appear, give the plants another week or so, then transplant to a
larger container. Hold the seedling by a leaf, not the stem - too
easy to crush. At this time, if you wish, supply a light, very light
dose of plant food, with a light watering in. That's all you need
until you have a PLANT to deal with. I start my entire (near enough)
garden every year, and I don't fertilize seedlings, just water and
light. I feed the plants when they go in the ground outdoors.
Second, you can think about splitting up the cukes and cabbages when
you see (again) true leaves. I would hesitate to TP seedlings prior
to this - beginnings are such dicey things. Give the seedling a
chance, then break 'em up when they put out leaves.
That's my story, and ahm stickin' to it!