Minecraft – play on a < 1.8 SMP version of Minecraft with the 1.8 without breaking something

No, ruins and villages are generated with the world, not actively built. If that changes in a future update, then it might not be required then, but for now all new geographic features require you to generate new chunks.

The usual way of doing this is just making a new world with all new chunks, but if you walk to places that you haven't yet, then new chunks will generate. (Potentially with new features.) It's still recommended that you generate a new world though since you might have to walk a while to get to new parts, the boundary between versions might not line up right, and biomes might shift(especially with the addition of a new one), but if you haven't explored much or really like it so far, it might be worth it.

To my knowledge there are no mods like this at all, and if there were any they would not be up to date with the 1.8 prereleases/snapshots. However there are a couple of tips to making easier use of the commands:

1. Hitting Tab will automatically fill out the coordinates of the block you are looking at.

   

   This is a much faster way of filling out coordinates than typing it in manually from the F3 screen. Hitting Space then Tab three times will correctly enter the X, Y and Z of the block your cross-hair is over.

2. Enter can be used to submit and "save" a half-completed command, then press Up while in chat to bring it back up and complete it.

   

   As the command does not have the correct syntax, you will get an error message when pressing Enter and nothing will happen, but you can still use Up to get the command that you entered back.

   Using this in combination with the previous trick allows you to move and easily enter both sets of coordinates into the command, without writing or even copy pasting anything.