Getting a balanced sample across many variables
If I understand correctly, you want a sample with along each feature the value below which a given percentage of observations in your population falls. If so, you might want to try something like this using np.percentile:
import numpy as np
import pandas as pd

data = [ np.random.randint(1,10,6) for i in range(20) ] # fake data
df = pd.DataFrame(data=data, columns=['A', 'B', 'C', 'D', 'E', 'F']) # fake dataframe

# 15 being your desired percentage
balanced_sample = [ np.percentile(df[col], 15) for col in df.columns ]