Ideal number of participants for evaluation of development approach / software architecture
"Is there any scientific research or paper that suggests a certain number of evaluators for such a rather theoretical approach?" 

There is a statistical method to calculate the correct sample size for an experiment.
This method is called a "power analysis" [1]. It calculates the minimum sample size needed to verify that an experiment's results are statistically significant.  This approach assumes that you have at least two groups.  A control group and an experimental group (i.e. control group uses the platform without your modifications, experimental group uses the platform with your modifications).

Thus, while there is a scientific process to calculate the correct sample size this approach is probably overkill for your study. Nielsen [2] states that heuristic evaluation is just a "cheap and quick" method for easy evaluation. As such, I believe you can justify a sample size of 5 by citing Nielsen.  However, if you do want to standardize your evaluation process and use a statistical approach I encourage you to look into power analysis. 

You may be interested in this online tool for calculating power.




http://en.wikipedia.org/wiki/Statistical_power
http://www.nngroup.com/topic/heuristic-evaluation/