つぷ (or つぷり, つぷっ) is not a new or rare onomatopoeia at all. You can google it (results may contain materials that are NSFW). It's basically a lighter variant of ずぶ (or ずぶり, ずぶずぶ, ずぶっ).
Only a very few creative writers try to make totally new onomatopoeias in their works, and there is no "rule" for such creative activities. Some can be understood as variations of existing ones (e.g., わきゃわきゃ), but some are similar to nothing. This page introduces some unique onomatopoeias based on their etymology (of course it includes めるめる).
Rules around commas are generally fairly loose in Japanese, and it's not uncommon to insert a comma before と. Here, that comma may help the onomatopoeia stand out from its surroundings. They could have used brackets (触れてみたら「つぷり」と) as well, but commas are less distracting.