for normal XRD measurements the minimum thickness of layer must be more than 7 microns. if the thickness of your film is less than 7 microns you can use Grazing incident X-ray Diffraction. in this method X-ray incident to the surface of sample with an angle about 1 degree and only the peaks of your coating appears in XRD graph. 

There are two types of XRD measurement modes, conventional and grazing incident mode. In general, for a good quality of XRD data for thin films, we always go for grazing incident XRD (GIXRD). For thicker films of the order of micron can give good quality of data. 

This is actually sample dependent.  According to ISO 17867:2015, the international SAXS standard, then the optimum thickness for x-ray transmission is given by to =1/μ where μ is the linear absorption coefficient of the material. The optimum specimen thickness corresponds to a ratio of the primary beam intensity with and without sample, Iout and Iin, of Iout/Iin  = e-μt = e-1 ~ 37%

Thus, the ideal specimen will transmit about 37 % of the incident radiation, and the specimen thickness can be adjusted accordingly to optimize transmission.  Ultimately the minimum thickness depends on the signal to noise in the system and the minimum detection limit above the background.

How much minimum thickness of heteroepitaxial thin film is necessary to measure XRD( theta-2theta)?

There is no general answer because it sample dependant.

By in-plane scan it cud down to few nm (ultra thin).

This actually depends on the transmittance and density of the thinfilm.

My own experience:

For epitaxial films, XRD peaks from 5 nm thick layers can be routinely observed with standard XRD equipment (3- or 4-axis diffractometers) if the substrate peaks do not fully overlap that of the film.

In the in-plane XRD geometry (or quasi in-plane), when the beam is at small angle (a) to the surface, the thickness of the layer is effectively increased from t to t/sin(a), i.e., for (a)~0.5° an increase is by a factor of 100.

A good popular overview is in the Rigaku Journal

https://www.rigaku.com/downloads/journal/RJ24-1/RJ240102.pdf

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Trying to learn what thickness is possible to study by XRD, I found a recent paper

by Chubarov et al (2018): " In-plane x-ray diffraction for characterization of monolayer and few-layer transition metal dichalcogenide films" published in Nanotechnology, Volume 29, Number 5