When entering finance with a technical background it is very typical to approach the challenges of economic life as a set of quantitative problems that just need to be mapped to existing algorithms. In fact the algorithmic part, if it comes into play at all, is at the end of a long chain of analysis and discussion that aims to create a rational context in which to start building useful tools.
To bring the issue home: standard credit scorecard systems are build for exclusion. You try to find the subset of the population that you can profitably lend to, the sole criterion being the risk-return calculation of the lender. Microfinance is about inclusion. You need to first understand what this mandate translates to in terms of requirements before you can start exploring the landscape of existing quantitative solutions.