I was annoyed that Britain, France, and Spain had to capture several regions before their own colonies joined them, so I edited the scripting.lua so that the player gets them at the start of the game and the AI gets them on their first turn (between the player's first and second turn).
The former colonies do not have any soldiers because adding soldiers involves editing the startpos.esf, which would turn this 34KB mod into a 65MB one.
To install download the scripting.rar attached to this post, unpack it, and put the scripting.lua in the 'empire total war\data\campaigns\main' folder (default location 'C:\Program Files\Steam\steamapps\common\empire total war\data\campaigns\main'). Backup the existing scripting.lua before you overwrite it.