There is no rule of thumb regarding correlation magnitude that we know of. Logically this would depend on the other correlations within the matrix. If two products are highly correlated but negatively correlated to other products, one would expect them to be weighted accordingly to their independent risk and return dynamics accordingly under the maximum Sharpe ratio for the portfolio. The correlation is only one attribute contributing to the optimal weighting as average return and standard deviation of returns for each product are also factors.
