Omics data collection and merging of datasets from different studies

Question

[BioQueen]

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!