Two comments
First, the solution to the US colonization problem may be a worse Civil War. Remember that the South wanted to colonize Cuba, Mexico and Central America BEFORE the war in order to expand slavery. A good balance might be making any external US colony either 1) a member of the CSA upon start of the ACW (to include any units, naval or land in the colony) or 2) the possession of any province past the 1836 start point increases the chance of a civil war (it was the expansion of the slave/free issue into the new territories that set the spark for the war). This way, the USA has the option to colonize upper botwanaland if it wants, but pays the price by having to fight a crapload of CSA troops with a stronger resource base when the war starts.
Secondly, when it comes to forces for the USA and CSA at the start of the historical war, the CSA actually had the advantage. Read Richard McMurry's Two Great Rebel Armies. In it, it notes that the South, in general, had a better militia/volunteer system than the North and was able to field troops quicker. It wasn't until Lincoln's second call for troops in 1861-62 that the Federal forces grew substantially over the Southern forces. Most of the battles in 1861 were fairly well matched (especially those in the Western and trans-Mississippi theaters). Maybe Vicky needs some sort of brake on US military expansion before the Civil War; lower soldier POPs perhaps or some sort of penalty for war weariness,etc. if a large army is created before the Civil War.
Just some ideas
Bob