Not an expert but hopefully its useful. If you have vast amounts of case reports or case series you could aggregate/pool the data as if it was a normal database and perform statistical analysis on it. You can perform forest plots on studies other than RCTs, however lower quality studies, however doing this would probably widen your SD/CI. If you have both RCT and other studies you could do subgroup analysis (ex a forest plot of all data and one with only RCTs.) You should start by reading available evidence to determine what you will be able to compare and not vice versa, studies are usually not that heterogeneous in their measured outcomes.

Thank you!

Unless you have fewer than 3 well-designed RCTS, I suggest you include both RCTS+Cases in your qualitative analysis and only the RCTs in your quantitative analysis to conserve the quality of the evidence. Or you can include it if there is a methodological explanation, then you can perform sensitivity analyses and subgrouping. In that case, you will need to employ the random effect IVhet model and pool the continuous data (mean/SD) as a weighted mean difference, and the binary data (event/total) as a relative risk.