Yes, I do see that, Strange it snaps to the edges when you are moving the image around, but not when resizing?
I'm thinking of another way also... create a 600x400 sequence and drop your clips into it. Then you can use that sequence in the edit. You can have just one, and drop in all the clips and just set edit points on that sequence in the main sequence. Or have a bunch of sequences @ 600x400. And just use those into the main sequence. Finally, you might be better off since they seem to be just pictures to simply editing them in Photoshop or any editor and make the size you want. Then bring them into PP like that - and with that, you can make a more extensive grid guide and snap the pictures to it.