It is best to "pin" some vertices in the cloth sim panel so they won't be affected by gravity. Select your clothing, enter edit mode, in Blender's data panel, create a new vertex group and the area you don't want the sim to influence. For example: Top of a skirt, collar of a shirt, cuffs of a sleeve, etc.

The collision on the character needs to be closer to the skin. On the character’s collision, change the outer to 0.005 or 0.001.

Remember: When attaching clothing, it is a trial and error process. Do not expect it to be exact the first time. Please refer to our tutorials. Try moving the top of the clothing slightly higher above the arms and shoulders. This is the usual area of issue. It will also help to add more ring/loops horizontally in the arm areas. This should close that gap. You could also move the highest shoulder areas closer together. Again, it is a bit of trail and error. You can delete the current model after sewing and restore the backup. This will keep the specific modifiers in-tact. Sometimes, undo-ing may have undesired effects.

If the clothing is too big or small, scale it down or up to match your character. If the character is smaller than one square on the Blender’s default grid floor, then the character’s scale will need to be increased.

~Rigging is offset when switching to edit mode?

Explanation: The above character's default 'resting pose' is in an A-Pose. The character's rig was put into a T-pose and then the clothing was applied to the rig. Because the character has already been posed before the clothing was attached, the clothing will now 'offset' and move the with it. As a general rule, anything applied to a rig will follow the 'resting pose'. 

Solution: Select the character, apply the rig modifier. Then select the rig in pose-mode, press space-bar and search for "apply pose as rest pose". Parent character to the rig and then clothing to the rig.

The clothing templates are designed to best work with T-poses. However, if the clothing is adjusted to your specific pose before hand, yes any pose should work. Make sure to read the above post regarding "Rigging Problems" for more info.

Select the clothing in Blender, go to the data panel and make sure the selected UV map is the one you want exported. When unwrapping each section (patterns, bump map, grunge), make sure you have the correct UV map also selected.