If you root, your phone will no longer be recognised by kies so if you want to update your phone to another official firmware you will need to use Odin again. You just download the file and flash it the same way as the cfroot kernel you use to root.
It is custom roms which you instal using cwm. There is a good guide to flashing checkrom here
CheckROM v6 - A Dummies Guide to installation
This can be applied to almost all roms as the method is the same.
I wouldn't recommend using that app from google play as ROM manager doesn't play very nicely with the gs2. Cwm will be installed automatically when you root with the cfroot kernel, which is the right one by the way.