Quite a few questions and I'll try to address them all as clearly as I can.
It depends on where you are adding the video. Within the portfolio items, it automatically adds in the videos to best fit the container. Some videos because of the aspect ratio it adds the black bars.
The same thing will happen in a page or post unless the video is inside of a smaller container as the video will stretch to fit the whole container its in. In this instance, you can use a shortcode for a column to give it a container and to make your layouts if you want the iframe to be a fixed width.
You'll need to be more specific on your frontpage question. The frontpage can have an almost unlimited number of layouts which can be controlled from the themes Template Builder. If you are using the dummy data, it will be controlled by the Frontpage template within that template builder.
Some of the more in depth options don't have any way to control them within a portfolio element however. Setting the exact order for instances isn't possible. The portfolio image size is controlled by how many portfolio items are in each row (the more columns the smaller the images are).