With sprites that have multiple parts (facings, animation frames, etc.), each part takes 1 index, so for example, handobs take 8 indices. If the last handob index is 735 and you start a new one at 736, you're replacing 7 of the facings for the handob referenced at 735. To not overlap, you should start at 735 + 8 = 743 for your index. For bullet sprites, it's more complicated; each sprite is 35 frames, but the ruleset reference is (sprite index) / 35. So for example bulletSprite: 12 is really referencing sprite frames 420 to 454, and bulletSprite: 13 starts at 455.
If you don't want to worry about colliding with FMP sprite indices, I'd suggest making a separate mod where you can start the index count where you want, as long as you respect the reserved space for vanilla sprites (128 I think for handobs, look up on
https://ufopaedia.org for the others).
Edit: Also, I would advise against putting stuff into the /path/to/openxcom/standard folder, as that's supposed to be the folder which contains only the files for making the vanilla game work, and any edits there would either be overwritten by or mess up an updated nightly version. This is another reason to use the user/mods folder, so all of your WIP can easily be contained and moved from version to version and installation to installation.