Distance less than freespace halfwavelength between patches in array?

Usually this distance is around 0.7 freespace wavelength, sometimes more or less, smallest i saw is 0.5. For my purposes i need it be less than 0.5 wavelength, patches still do not overlap, array factor looks ok. Any problems with such tight patch placement? Distance between patches center would be around 0.3...0.4 freespace wavelength

Not a problem at all :)

The only thing to be aware of is that the final phased array beamwidth is a function of the overall array aperture. i.e. a 3x3 array with 0.1 wavelength spacing between antennas wont achieve the same narrow beamwidth the same 3x3 array array will with 0.5 wavelength spacing.

There is an other rule that you can follow. To get the best antenna correlation, the mutual coupling between antennas should be more than 10dB at working frequency.