For the most part there aren’t any bad links, just links that Google discounts or doesn’t consider altogether. For what you are talking about there is no concern for having multiple links in signatures on forums, especially if you are indeed the one discussing a topic.
As for multiple links on one site, there is a degrading effect. One link gives you most of the juice that Google will pass from one site to another, but you can squeeze a little bit more juice out of a website with a second and third link. Eventually there is no more juice to be had from a single domain. Of course if a forum topic becomes popular enough that other websites link directly to it, that page with your link on it will gain more juice and make your link even stronger.