Click the red circle on a checkered ground button to activate the snap-to grid function. This will link the tiles seamlessly, at least when you also activate the next button (snap to angle) and only work in 90° angles.
As for working with the tiles, it's somethng you need to work with to get used to. use the preview window to see the tiles before placing them in the render window, and have a look at how they were used in vanilla areas. Best set "bAllowMultipleEditors" in SkeyrimEditor.ini to 1 and open a second CK where you can rifle trough the exiting areas, move objects etc, without adding those changes to your mod.
You can group-select using the "rubberband" mouse-selection, or by holding CTRL and selecting several objects, or a combination of both. Permanent grouping (i.e. until ungrouped) was possible in the fallout editors, but no longer in the CK.