Most often, but not always, there is correspondence between picture aspect ratios and preview aspect ratios. You can be assured that at least some of them are classic 4:3 ratios (e.g. 640×480). Support for 16:9 is also widely available.
The screen may have different aspect ratio. To fill it with a camera image correctly, it is widely accepted to add black margins (same approach is used in YouTube). Alternatively, you may crop the camera image to fill the whole screen.
Note that the screen size (as reported in tech specs for the variety of devices) is not always actually available for your image display. For example, the system menus, and title bar, etc. may take their share of the screen real estate. The immersive mode is available on some devices, and its behavior depends on the system version. You may expect that future developments of Android, e.g. second screen support, will make the game even more interesting.
So, the answers to your specific questions:
is it best to use a match_parent or fitXY for the FrameLayout size and only work with two variables, the preview size and picture size? – no.
is it always the case that there will be matches in both sizes? – yes, but maybe this size is not optimal.
Even if your camera supports different “wide” picture sizes, 1280x720 preview and 2560x1440 picture may be the best match, both precisely at 16:9. Usually, the quality of picture at small size is not significantly better than preview at that same size, so you may choose to save the preview frame if 1280x720 is what you really need.
If you can forgo takePicture(), your app will be much more responsive.