United is not maxed out on 76 seaters, but they will be maxed out on 70+76 seaters once Shuttle America has placed their 50 E175s in service (replacing the Q400s), unless they buy CS100, E190 or E195 aircraft for mainline.

In order for the total 70+76 seat aircraft to exceed 255 following the Small Narrowbody purchases, they have to reduce the number of 70 seaters to no more than 102. By the end of 2015 they will still have 138 70 seaters in the fleet (100 CR7s and 38 E170s)...so look for United to remove at least 36 more CR7s in 2016/2017 if they want to grow the 76 seaters to the max.

Bear in mind that if they keep the 70+76 seaters to a max of 255, then they don't need to buy the Small Narrowbody aircraft, so they have the option of replacing CRJ700s (or E170s for that matter) with E175s without needing to buy anything for mainline. They would be limited to 153 76 seaters and 255 70+76 seaters in that case.

United is not required by scope to reduce the # of 50 seaters until they have more than 153 76 seaters in service. Even with Shuttle America's E175s that will only put them at 120 76 seaters, so they are not up against that yet.

The only other consideration is the number of block hours flown by UAX as a percentage of mainline single-aisle block hours. I have no way of figuring that part out, so I will assume it is not limiting at this time. Mainline adding A319s would obviously allow United to increase UAX block hours under this section.