First off, here’s a useful general rule when building in patterns: you can make anything fit into anything else if you multiply their sizes together to get the lowest common multiple, as long as the edges match up. For example, say you have a 3x3 floor covering and a 4x4 tunneling tool, you can multiply 3x4 to get a width that both tools fit neatly into.
Of course, if the edges don’t match up then it’s significantly more difficult – you can’t dig a hole that’s half a block wide or deep. In that case, you’d have to turn to modding to create an “offset” version of the tile which lines up neatly with the block edges rather than overhanging halfway.
As for your second point, I’d love to see some simple floor tiles for making easy floor designs with! This can definitely be done as a mod, and it would be an awesome way to decorate roads (which are, AFAIK, the only surfaces in the game which have a special feature when hearthlings walk over them.) Of course it’s possible to build your picture out of full cubes, and you can use mods to get more building colours; but it would be wonderful to have floor coverings that preserve the “ceiling” of rooms underneath… just in case we ever get the ability to look upwards from the hearthlings’ perspective!