Is it possible to only move clips or adjust in/out at 10 frame intervals by snapping (so just 0, 10 & 20)?
There is no timeline snapping other than holding down Shift while moving the bars around, but you can skip ten frames by using Ctrl+Shift+right/left arrow keys.
I would also like to avoid having to press Shift while scaling multiple images/videos. Is there a way to toggle on a lock of these ratios?
Scaling values are locked/ constrained by default, you're just using a abd workflow by doing it interactively. simply parent all elements to a Null, then numerically scale that Null by scrubbing the value and the child layers will inherit that. You can also nest the layers and then simply scale the entire comp. Likewise you can use simple pickwhip expressions to connect values across layers/ comps... Many ways to skin a cat.
I'm not sure I follow. Scale is in percent, not absolute pixels, so the aspect ratios and overall dimensions do not matter unless you need to fit things specifically to other pre-determined pixel sizes. The only difference would be that of course 10% of a 4k image is different from 10% of a HD res file, but the math is easy enough....
It would help a lot if we knew what you were trying to do. Editing the scale value in the timeline keeps the aspect ratio correct as long as scale constraints are turned on. Scale is centered around the Anchor Point. Move the anchor point to the top right corner of a layer and it will grow and shrink from the top right corner.
If you have a bunch of layers distributed around your frame and you want to scale maintaining the relationship between their position you cold parent them all to a null and scale the null.
If you have layers that form the pages of a book and you want to scale up the book you could also use parenting or you could put the anchor point of the left side on the center of the right edge and the anchor point of the right side on the center of the left edge, then tie the scale of the left to the right with a simple pickwhip generated expression. Now both layers scale from their appropriate edges and the "book" stays together.
You could also change all layers to 3D and add a camera. Moving the camera around would maintain the relative position and the movement would actually look better than scaling. Read up on exponential scaling in After Effects to learn more.
The long and short of my answer is, tells us or show us what you are trying to do and we can help more.