About Mod Conflicts:
The first line of defense for mod conflicts is Sims3Dashboard. It is found at Mod The Sims. Google it. Get it. Put all your individually tested mods that you know you would like to use, along with your core mod, into your Mods folder. Then run Dashboard. It will highlight the all the conflicts for you, and tell you which particular mods are in conflict with each other. When mods conflict, you can only keep ONE of them. Other mods in conflict must be removed. Which mod or mods you remove is entirely up to you.
Make your hard choices. Do your weeping. Then get on with your simming life. ;)
The above may be all most people need or care to know about conflicting mods. But just in case you are the curious type, I will say more about it. Essentially, modders are changing bits of code that modify the basic game. There are only so many bits of code. This means that modders who want to influence certain game aspects ALL have to modify the identical bits of code that relate to that particular game aspect.
Since the game can only use one bit of code at a time, if the game sees two modifications of the same identical bit of code, it creates an error. The game may sporadically choose first one and then the other modification for a bit of code, thereby doing something entirely unexpected and erratic. Or the game may disregard the bit of code, in a fit of indecision. Either way, you have instability and possible corruption. CONFLICTS ARE A BAD THING, and must be corrected.
A good modder will tell you what type of modification they have made to the code (XML, scripting, override, etc), and they should also tell you which bit or bits of code they have modified, if applicable. So, once you get to know a thing or two about mods, from personal experience, you will begin to recognize which mods are likely to conflict. (If a modder doesn't tell you what bit of code they changed or what kind of mod it is, don't be shy about asking. They have no reason to hold back that information, and it is something you do need to know, for responsible modding.)
Obviously, the more mods you have, the more likely it is you will have conflicts. Don't be a mod-aholic. Use only the mods that matter the most with how you play your game, and resist the temptation to throw in every mod that looks even mildly interesting.
What the Dashboard is doing, behind the scenes, is comparing the bits of code that have been changed/modified, and flagging mods that change or modify the identical bits of code. The program is a good one, and generally comprehensive. But it may not catch every little thing that could go wrong. So, if you notice instability in your game, you may have to troubleshoot until you find which mod or mods are causing the instability.
The best way to troubleshoot for mod instability is to take out half your mods, and test. If you come up with no problems, then try the other half of the mods. If you see a problem, again divide that half of the mods in half, and so on, until you locate the culprit.
Bear in mind that a problem may only show up with a few mods operating in tandem, so if both halves show no problem, then you have to divide all your mods in quarters and put the first and third quarter together, and then the second and fourth quarter together, and then the first and second, and then the third and fourth, and every variation possible, until you find the variation that kicks up the problem, so that you can isolate the real conflict or conflicts of a set group of mods that cause trouble when working in tandem (but are fine, otherwise).
Did I mention that working with mod conflicts is not for the faint of heart. ;)
Throughout your entire experience with mods, you must keep in mind how useful they are, and how much of a difference they will make in your game. With that in mind, determine that you will make it work, one way or another. Keep a good attitude, and with some effort and head scratching, you will eventually cobble together your ideal game. :)
More to come, so stay tuned ....