We are designing research in which we will explore the perception of users (C current, F former, and P potential) on a product. We apply the same questionnaire for each group adapting (verb tenses) some questions depending on user group (C, F, and P). We use five-point Likert scales, multiple and single choices questions to measure intention to use the product, and willingness to give feedback, among others factors. We adapted some scales from previous studies like Lajunen and Summala  (attitude towards driving).
We want to compare the results from each group to conclude, for example: current users are more willing to get feedback using the product than potential users, former users are more willing to share their personal information using this product than potential users, etc.
After running a screening survey we identified 30 current, 70 former, and 400 potential users of the product that we want to evaluate. Thus, we could recruit a maximum of 30 current users and 70 former users (the best-case scenario).
We want to know if recruiting different sample sizes for the three groups (C current, F former, and P potential users) could add value to our analysis, for example running the survey with 30C/70F/70P, or 30C/70F/200P. In case of deciding to conduct the study with different sample sizes for each group:
-> How many participants per group should we recruit?
-> How could we analyse/compare the output from a small sample (less than 30) with another big sample (more than 100)?
-> Could we compare the analysis from a parametric test with a non-parametric test?
 Lajunen, T., & Summala, H. (1995). Driving experience, personality, and skill and safety-motive dimensions in drivers' self-assessments. Personality and Individual Differences, 19(3), 307-318.
We also appreciate getting any relevant literature.