Min number of bacteria to screen to characterize 2 and 3 way competitions

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mycorr10
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Min number of bacteria to screen to characterize 2 and 3 way competitions

I am doing competition experiments with 2 or 3 nodulating bacteria at a time for nodule occupancy. We test the initial inoculum to figure out the starting proportions to correct our numbers after harvesting nodules  but my boss wants to drop that part of the experiments- stating that the small sample size we use to screen the population ( 20 or so clones by PCR) is not enough to be representative and it's probably close enough to the 1/2 or 1/3 we intended to start with.

My question is this-  what sort of sample size would be advisable to determine the proportions of the starting population given that there are two or three bacterial cultures mixed together? Also is this step - testing the initial inoculum to correct for imbalances in the proportions statistically advisable or does it introduce more bias?