I'm planning on getting a NES soon. (Nothing to do with the input file rule, though). I have a question, though. Would an unofficial multicart (I mean, the 1 in 500 carts and 1 in 140 carts and stuff) be fine to run, or do I need an official game or official multicart (like the SMB/Duck Hunt)? If I have to get a real game cart, it's fine, but it'd be a little less money I would have, and I'm poor :P

The legit game cart for Duck Hunt / Mario is super cheap - I think my local used game store sells them for $10

I have one of those carts, and the 143 in 1 cart works completely fine for that, because it is just ROMs loaded on the cartridge I think. You should be fine or just do what @Darpey said and find a cheap one online for $10 or less

Yeah, I have a friend that's an Elite Pro member on GameStop and I'm getting SMB/Duck Hunt for 4 dollars there (plus a discount code, so probably closer to 3 dollars)

We’ve found the unofficial multicarts commonly do not have the original ROM.

i believe the 143 in one carts do, since i have done an fpg on the cart.

I’m not sure how being able to execute the FPG on that definitively means the ROM is unmodified.

That isn't a good way to tell if it's unmodified, I've done FPG on many ROM hacks.

True