yeh you can mix both in same galon just add the grow big first and mix well before adding the other.and yeh if you veg for until you see pre flower then you can determin sex before going into flower pre flower usually show after the 5th or sixth node but be carefull with the nutes let the plants tell you when they are needing fed cuase the soil you have has all the food the plant requires and the leef tip curl is telling you they have had there full. be safe the postman.