Better risk losing 80% of you fan base 'now' than if you were to lose it at 500 fans. ;)
That being said...:
https://www.newgrounds.com/wiki/help-information/content-submission
https://www.newgrounds.com/wiki/help-information/terms-of-use/art-guidelines
GrapheneKid
Taboo is kind of a subjective quality, especially with fandoms. Like in some fandoms, shipping two specific people together, even if they are ethically matched, is unpopular. It just depends on whether the thing is taboo for ethical reasons or just taboo because it's extremely unpopular.
For me personally, as long as you're not into loli or shota or anything like that, you're alright.