You really should get another duck ASAP. It's not healthy to have just 1 duck. They will always follow you, as they've imprinted- they need another companion- a duck or goose.
Mallards have 1 eye stripe, and Rouens have 2.
Mallards are seasonal layers, so will lay in spring and summer. Rouens lay more eggs and lay spring, summer, and fall.
They start quacking as early as 5 weeks, usually around 6-8 weeks though. You won't know gender until they start quacking.