For any cross, people who know genetics can make reasonably accurate predictions about how the chicks will look. Occasionally a chicken will have a gene that is not expected (and may be "wrong" for their breed), and of course that can make the predictions wrong.My main question is this: I'm hoping to start hatching chicks later this year, raising them a lil while, and selling them as pullets. But let's say I was gonna cross two completely different-looking breeds, like a buff orpington rooster with a speckled sussex hen... would they actually be half-and-half color-wise, or inherit most of the genes from their mom and only some from their dad?
If the chicks inherit the same gene from each parent, that's what they show.
If the chicks inherit a dominant gene from one parent and a recessive gene from the other parent, they show the dominant gene.
Some genes interact in various ways, producing chicks that look much different than either parent.
To address your specific example:
From a Buff Orpington rooster and Speckled Sussex hens, you should get chicks that grow up with a lot of gold and/or brown feathers, probably with some black bits but possibly with white bits instead (depends on whether your Buff Orpington rooster has the gene called Dominant White, that turns black into white.)
They will not show the specific patterning of the Speckled Sussex, because the mottling gene is an important part of that pattern. The mottling gene is recessive, so all the chicks will inherit it from their mother but none of them will show any effects from it (maybe a tiny white tip here and there on their feathers, but often not even that.)
For any trait where the parents have the same genes, of course the chicks will match them both. In this case, it would include single combs and white feet/legs. The chicks should not have any of the fancy traits that some other breeds show (feathered feet, crest on the head, beard on the face, silkie or frizzle feathers, extra-long tail, 5th toe, other comb types, etc.)