The intensity of the test charge should be very less cause it will disrupt the field. Question is will it matter even if it disrupts the field? Consider 2 point charges q1 and q0 (take this as the test charge), as the intensity of 2 increases, so does the magnitude of force that applies on the 2, what does this mean?...it means that that though the field of the 2 is getting disturbed, the attractive/repulsive force are not; so the attractive/repulsive forces should be independent of the degree of disturbance in the field which they mutually cause. So why does the test charge need to be small, it'll give the same result. Lets analyze what I've said by the definition of E.F - E = F/q0 Considering a constant E.F, as q0 increases, so will F ( Coulomb's law) and so the ratio will remain constant..............so the value of q0 should not matter at all! In fact how do we even come to the conclusion that under addition of new charges within the field of another source charge, the E.F of that charge gets disturbed? Cause if a test charge is put in this scenario, the force on it will be altered?...well it's altered cause of the other charge, that is the force on this test charge will be a resultant.