Simulations of 3 He-4 He mixtures with negative separation ratios in two-dimensional containers with realistic boundary conditions and moderately large aspect ratio ⌫ are described. The system exhibits a large variety of states with complex time dependence including intermittent wave localization and chaotic "repeated transients." Steady but localized states are also found. Particular attention is paid to the transitions that occur for ͑R − R c ͒ / R c Ϸ ⌫ −2 , where R is the Rayleigh number and R c its critical value for the primary instability, in order to clarify the gradual transition from a small number of active degrees of freedom ͓͑R − R c ͒ / R c Ӷ⌫ −2 ͔ to many active degrees of freedom ͓͑R − R c ͒ / R c ӷ⌫ −2 ͔.