1. The norm is to set LVL CHG & MCT as soon as the a/c is clean.
2. The info'below I pass on from a Chief TC in a previous company.
TOGA and LVL CHG have different bias; the former is more towards climbing and the latter more towards achieving the speed. However, at high weights TOGA, while trying to climb, might find the acceleration too slow and guide level flight. I believe that TOGA will analyze the rate of acceleration and guide an attitude to accelerate at a minimum rate while climbing as best as is possible.
This was also the reason, contrary to Boeing, why we did not select the autopilot at acceleration altitude because it would cancel TOGA, and you would lose this computed performance.
On the otherside, level acceleration is allowed at all weights and this could be a SOP. Some companies use V/S and select whatever seems sensible on the day. This flies in the face of SOP philosophy as it could lead to very different opinions and techniques during a critical manoeuvre.