Carla is an unusual character, because she has custom morphs. Most of the characters you see in the stores and the galleries use the standard morphs. The ones DAZ sells. That makes it a lot easier. Many clothes come with the most popular DAZ morphs already built in, so you don't have to mess around with Morph Manager. (Carla is a great character, but I really don't recommend her for beginners.)

Yes, ideally the shirt morphs should be the same as the character's morphs. You may have to tweak it a bit, especially in extreme poses.

But generally, you don't adjust the morph dials in each body part. Instead, use the dials for the body of the character.

You can change all the body morphs at once, by using copy and paste. Select the body of the character, click on the top line in the parameter dials box, and hit Ctrl-C. Then select the body of the dress, click on the top line, and hit Ctrl-V. That will set all the morphs dials in the dress to match the morph dials in the figure...but only if the clothing has all the necessary morphs. Not all clothing does.

It's easy to change the colors of an object in Poser, so find a dress you like, and turn it white in the material room. If it has a texture map, delete it. Then set the Diffuse Node to the color you want.