1. It's a good habit to bake your mesh's shadows from Maya and export it in Photoshop and used it as shadow base for painting darker parts.
2. Try to paint as big texture as your pc can handle, as you can always go smaller for texture size, but if needed to show more details can never stretch it out bigger. For my highpoly models I usually paint at least of 4k - 6k maps.
Shading :
For shading I used a basic Blinn shader with color, bump and specular map attached to it. For leather jacket I used Maya's procedural leather texture in bump slot. Though the texture was handpainted and when mixed with procedural bump, it worked well. When ambient occlusion was not included in Maya, I used dirtmap plugin for every shader. It gives softer shadows and gives more depth to scene. Though it's a bit expensive in terms of rendering.
