1. Ya, it can be done. But it is going to be little tricky. First please copy the layout to your wordpress theme to modify it. You can follow the Topic 7 here http://www.masonrylayout.com/documentations/. After that you will get the new layout location. IN that folder there will be social icons, you can replace them with the ones you have.
2. In Masonry layout, it doesn’t make difference to have different image dimension. The width will be same and height are auto calculated based on width. If you need to make it exact size, you must need to upload the images of same size for all posts.