If I'm on the Tarifa Constante, are my surplus credits compensated at the fixed price I pay?

No. Even though your consumption price is fixed with the Tarifa Constante, surplus compensation is always calculated at the real market price (OMIE minus deviation costs).

These are two separate concepts: the price at which you buy electricity and the price at which you sell your surplus. The latter always follows the market, regardless of which tariff you have contracted.