Omics data collection and merging of datasets from different studies

Hi!
I’m going to do omics analysis on skin basal cell carcinoma. I’m currently trying to find publicly available data to use to make a data collection. I have looked after RNA-seq(high throughput) in GEO and found some studies like for example GSE156171, GSE128786 and GSE125285.

I’m going to combine datasets from different studies and I have a few question regarding this.
1) What do I have to think about when choosing datasets from GEO(like what type of high throughput RNA-seq can be combined, for example can I combine lncRNA with mRNA or what about miRNA and scRNA?)
2) And how do I combine different datasets from different studies?
3) Also do you recommend other databases?

Thanks!