Some random tips for starting out with Eurorack...
Have a realistic budget for it. Then double that

Same with the case you get. It will get bigger, so buy one that is big enough to begin with. Think about everything you want, then again double the size

However, start very slow with modules! (Get some blank panels to cover up the empty space in the rack.) Just get a few for a start and work them with the XS. After you get to know them well, you will start to notice the functions that you are missing. That will tell you about the subsequent module purchase.
Don't blindly follow all the hypes, there's like 4000 modules as of now, so you will have to make choices. Try to avoid impulse buys (except for the rare stuff, hehe). Of course, some hyped modules are very good, but you will figure out what that is if you take some time to research.
If you want to inject line level signals into the modular, you will need an amplification stage ("input module"). The internal levels are higher and you will be rather disappointed with a lot of modules when you don't have enough signal strength. For going out of the rack, use either an output module or just an attenuator or VCA.
Regarding your question with the 15V, that shouldn't matter. Whatever input you will send it to will just clip it at 12V (according to the power rails). It would have to be a very poorly designed module to be harmed by that.
And finally, be careful with the power supplies! Double check all power ribbon connections to be accurate before powering on! Make sure your PSU has enough juice for the modules you have. Use plastic washers to avoid rack rash (and have plenty of washers and screws). Ground yourself on a radiator before handling modules (i.e., PCBs) and don't store the modules on carpets when rearranging. Have more patch cables than you need. Don't patch outputs to outputs and don't mix with passive mults.