"Advanced Fishing" mod is an open source Minecraft mod released under version 3 of the GNU Lesser General Public License. What this means is that the source of this mod is publicly available and you have certain rights with respective to the code. Feel free to read up more on Open Source Software and what the GPL/LGPL give you in terms of your rights to software released under that license, but the short version for you folks here is:
you have the right to have access the source code of the mod.
you have the right to be able to edit/use parts (or all) of the source code provided that you provide proper credit to the original authour(s).
you have the right to distribute the source code and/or compiled versions of the source code.
you have the right to use this mod in Lets Plays/YouTube videos however you see fit (monetization, for fun, etc) as long as you provide credit to the original authour(s)(a link back to this thread for example).
"Advanced Fishing" modis an open source mod and released under the LGPL v3, that means you have the right to include this mod in your modpack.
Originaly this mod was created as a part of the "Gravestone mod - Extended" to make it able to fishing in lava and toxic water. But those features required a lot of changes in fishing mechanics, so I decide to completelly rework fishing mechanics, split it to an standalone mod and add new catch for different types of biomes to make fishing more interresting and useful. This mod provides an API so everyone can add an own types of catch for any types of liquids.
At the moment this mod still in development, so it is not possible to fishing in lava - there is no proper fishing rod for it. It will be added in next release.
When you say this overrides the fishing mechanics, what sort of changes are we looking at?
Is it still the mini-game where we wait for the splash, or something else?
Vanilla mini-game wasn't changed. The main difference is that your catch depends on dimension, biome and type of liquid in which you are fishing. Also I'm planning to make it dependent on the size of the liquid pool in which you are fishing.