The values will be converted to the most restrictive class so preconvert the integers to double:
Q = cellfun(@double,Q,'UniformOutput',0);
Also rather than the for-loop you can use the comma-separated list expansion of the cell array:
P = horzcat(Q{:});
Of course if you would like the best of both worlds (floats and integers) and would like to have them in a non-cell array (and you have the Statistics Toolbox) then you could use a dataset
Q = {uint8([1;2;3]),uint32([4.000;5.000;6.000]),[7e+04;8e+03;9e+02],[0.1234;0.34;0.4]};
D = dataset(Q{:})
