Using Google Scholar for meta-analysis
Using Google Scholar is usually discouraged as your primary database in systematic reviews because the searches are not reproducible (recommended are databases that provide reproducible searches such as Web Of Knowledge, if you have access, or CrossRef). Using it as one of the secondary strategies, is not necessarily problematic but the issue of reproducibility still applies.

The reproducibility of your search strategy is important because it is your method for the systematic review (in your case a meta-analysis) and determines which effects are included.

Also note that if you already consider your literature to be scarce, Google Scholar might make it more difficult to find sources, because of the low specificity of searches (Google Scholar tends to have way more results than are relevant to the search on the later pages).