Mod Compatibility checking program?

[JCourtney]

Does such a thing even exist that is up to date enough to work with 1.4.6/1.47 mods? When you add in mods from outside the FTB packs, do you need to add it to both the client and the server?

 

[slay_mithos]

1. Why would you even need this? As long as a mod doesn't make changes into the main Jar, it is technically compatible. Even safer if it uses Forge.
Some mods that make big changes to core gameplay, like changing completely the world generation and such, can make it really hard to play with some of the mods though, but they are still compatible in that they don't crash the game.
One exception is when a mod is based on an other, using the API this mod provides, because you need to double check the versions, or else you could encounter weird bugs.

2. Well, it depends on if the mod affects the multiplayer or nor, but most mods do indeed need to be added to both.

 

[JCourtney]

To check for item ID conflicts etc.

Is it possible to run Forge and Modloader together?

 

[slay_mithos]

Modloader is useless, as Forge already includes its own, and both change the same files.

 

[whythisname]

If there is an ID conflict it should show up as you try to launch the game/a world. Maybe not exactly which IDs/mods, but it should throw warnings which should be enough to tell you something isn't 100%.

Also, as far as I know no harm can come from just throwing in mods and trying them out. The worst that can happen is that Minecraft crashes and the worst issue that can cause are corruption issues in your world, but the chances of that are very small and are more likely to happen because of poor coding in 1 mod (like Advanced Machines used to have) than conflicts between mods.
Neither of those are huge deals if you ask me. Sure world corruption is annoying, but 9/10 times removing a mod and/or opening and saving the world in MCedit can fix it again. And losing a MC world isn't the end of the world, especially if you make backups you lose little to nothing. And then there are cheats in the case you lose any items or a world and you don't want to do a lot of mining again.

So yeah, just throw in mods and see if they work or raise conflicts. If they don't, you can be 99% sure everything will be fine.