i manage to figure out some issues and find out that the forced insert of featured image is causing the thumbnail size problem
Choosing whether or not to include the featured image in a SimpleViewer gallery should not change the size of the thumbnails in the gallery. If you display the featured image, the only thing that changes is that there is an extra <image> entry in the gallery's XML file.
As I noted above, I think the problem is with your WordPress theme. In the Mobile Player (which your gallery uses), all thumbnails should be displayed as the same size regardless of their actual dimensions. The fact that your thumbnails are displayed at different sizes is more than likely to be caused by CSS rules within your WordPress theme overriding those within your SimpleViewer gallery.
Try temporarily changing your WordPress theme to a default theme, such as Twenty Thirteen (and clear your browser's cache before reloading your gallery's web page), to see if this makes a difference.
With regard to your test post, settings relating to the image source cannot be set as defaults. Only settings relating to the layout of the gallery can be set as defaults. When you create each individual gallery, default layout settings will be applied but you will need to chose the image source you wish to use (and any other image-source-related settings).
I have checked the functionality of the 'Include Featured Image' checkbox and it works OK for me.
Try the following:
(1) Create a sample gallery with WP-SimpleViewer and select the 'Include Featured Image' checkbox .
(2) Click the 'Add Media' button and attach two images to the post containing the gallery.
(3) Click 'Set Featured Image' (from the menu on the left) and select 'Uploaded to this post' from the drop-down menu.
(4) Highlight an image and click the 'Set featured image' button (at the bottom right of the media window).
(5) Publish the gallery.
You should see both images in the gallery.
Now do the same again (with a new test gallery) but this time, deselect the 'Include Featured Image' checkbox whilst creating the gallery in Step #1 above. You should see only one image in the gallery. The featured image should not be included. This works for me. I have just checked it.
the first 4 images in the gallery is not supposed to be included
In a 'Media Library' gallery, all images attached to the post containing the gallery will be displayed in the gallery. Only the featured image can be excluded.
the post is in a 5-row table, first row i build the gallery and the
4 images are insert to the 2nd to 5th row independantly
I am not sure what you mean by this.
i try to insert a blank row between the gallery and the images
It is not possible to insert space between the rows of images within a Mobile Player SimpleViewer gallery.
should i never place images behind the gallery?
It sounds like you might be clicking the 'Insert into post' button in the media window.
If you click the 'Insert into post' button, the images will be inserted into the post as well as included in the gallery.
All you need to do is attach the images to the post for them to be displayed in the gallery (wait for them to finish uploading and then close the media window with the cross at the top-right).
SimpleViewer Support Team.