if you took the bioballs out and are using liverock rubble in chamber 2, why not fill all 3 chambers up close to the top? With this small of tank the more water volume the better. Also consider this, with your rock and sand you probably only have around 10g of water even though its a 14. JMO, hth
on the maxijet, you should be able to just disconnect the hose on the back of the wall and pull the stock pump out. The stock pump has little suction cups so you might have to tug a little. Once you have it out just replace the stock pump with the maxijet and then set it back down in the chamber and reconnect the hose and elbow to the outflow on the wall. Simple as that.