Armaholic is a well known website in the Arma community that is known for it's curating and distribution of mods since the launch of the first game. It's also a great place to see where new mods and updated mods are published as they are featured on the front page when they are updated or published.
- Modifying an existing mod
When it comes to using something that someone else had already made, it's fair to use and modify it however you want as long as you do not publish or redistribute it publicly. You are able to modify a mod and share it among friends but you are not allowed to publish a slightly different version of an already existing mod with some personal changes you made.
When users make suggestions it's always a good idea to respond positively. You do not want to discourage people from making suggestions or feedback as they might end up having an idea better than you thought of, or thought of adding something you didn't think of. Using my mod I made for example, most of the features in it at this point are features that other people have suggested. If the suggestion is too far from what the mod is meant to be, it's a good idea to respond along the likes of "It's a great idea, but I don't think it's in the right direction for the mod". Last but not least, if someone is taunting or bullying you, ignore them. On the Workshop you can ban/remove comments that aren't productive, and Armaholic and Bohemia forums have moderators that will take care of any harassment themselves.
When updating the mod it's best to include as much information about the update as you possibly can. In fact, it's best to note down changes to the mod as you do them so you don't forget. This allows users of the mod to troubleshoot better if something is broken to determine if your mod is the issue or not more easily. There are no strict version number conventions in the modding community but it's best to keep it simple. (Version 1.2, 1.21, 1.22, 1.3, etc)