Currently I just have mine piled together. I may opt to epoxy them together. Unfortunately if i do this, I won't be able to remove the rocks for cleaning and/or removing fish if needed. Another idea is to drill into your rocks and insert carbon rods into them so that they become more stable. Since no glue is used, you can remove the rods and then remove the rocks as you see fit down the line.
As to adding additional live rock to an established tank; there may or may not be any die off to cause a mini cycle. If possible, I'd suggest to buy your LR
from an LFS
and take it home with some salt water in a bucket\container. This will reduce die off to a minimum. If you decide to buy LR
online you will need to be careful. Die off will most certainly occur and how much will depend how long it takes to ship to your house. You can do overnight/2day shipping but you'll be paying up the wazoo for that since LR
is pretty heavy.