The WeeBr's are a bit twitchy by default of their design ie. light weight, short wheel base etc, which you will get with any small bike but IMO the next cause of instability is the budget suspension. Not so much of problem on perfect smooth roads but high speed cornering on anything other than that I find can be down right scary at times.
Now THERE'S a waste of $. I still can't believe someone wanted to put a fat tire on the back of their weeBR now this!
Swing-arm extensions are for high powered drag bikes that want straight line stability for racing. If you are feeling unstable day to day you need to practice at slow speeds using your rear brake or get used to the way a bike feels on skinny tires going through corners. A dragged out back end will compromise your handling and look ridiculous.