Even crediting the original artist or source does not grant you license to use their assets/art/work. They have to expressly give you that license, whether it be something as defined as a Creative Commons or as basic as 'yes, you may use it with credit'. And as noted, PokéCommunity has explicit rules against using assets without that license/express permission. So doing just that would not be a wise idea.
A general suggestion: If a project has assets that you would like to use but there is no indication that said assets are not available to use publicly, just ask the poster. In some cases, sprites are actually available for public use but are just not easily accessible (for example, the original uploader left the scene years ago and the source website is gone). But please make sure not to pester the poster. If they don't answer, take that as a 'no, you don't have permission to use this resource'. Additionally, if a project explicitly details that custom assets are not available for use, don't ask and don't use them. The key is being respectful. For example, I get requests to use some of my old sprites for commercial purposes fairly frequently. I politely point to my license and explain that no, I do not allow commercial usage of my sprites under any circumstances.