crowdmap is a map editor for pokecrystal. I personally use a hex editor plus some Python scripts to render maps and tilesets. (It's easier than it sounds. I design maps in paint.net, so it's not much slower to enter the tile IDs in Hex Workshop than it would be to place them one by one in crowdmap.)
You don't need some 10 GB program. Just Cygwin with the basic packages plus make, git, and gettext (I've installed many more packages and it's still only 4 GB); also the python package for crowdmap (around 100 MB); rgbds (500 KB); pokecrystal itself (60 MB with full Git history, 10 MB if you just download the current code); and crowdmap (130 KB). Total size: probably still under 4 GB.
To start with pokecrystal, follow the INSTALL.md instructions and make sure you can build the unmodified ROM. Then browse through the source files, get to know where different things are, study the syntax for event scripts (in the maps/*.asm files) and raw assembly code.
Once you get the hang of it it's much easier than using particular tools for every thing, and much more powerful.
I recommend making a GitHub account so you can fork pokecrystal, and instead of running "git clone
https://github.com/pret/pokecrystal" when you get to that part of the instructions, run "git clone
https://github.com/BronzeSwagger/pokecrystal". Then you can push your own changes to GitHub and have a backup of your project, with a change history that makes debugging a lot easier.