Is it possible to search a tag and instead of getting a pool of images, their IDs rather?

I'm not entirely sure what you mean, but for every search you can get a parsable response using the API instead adding .json (like change /posts to /posts.json, keeping everything else the same), which includes IDs.

So you're saying if I use E621.net/posts.json/XYZ then it'll also return the IDs?

jakobthedecidueye said:
So you're saying if I use E621.net/posts.json/XYZ then it'll also return the IDs?

Well, it would be e621.net/posts.json?xyz (where xyz is a search, same as the rest of the URL when you do a search normally). That gives the result of the search in JSON format, which does include post IDs. It's not something you're meant to look through manually though, you'd need a tool to parse it.

You should look at the api help