let's say I send eth to a second address but give a low commission so I don't overpay.
I don't really understand but i suppose you can try to user Smart-Contract address where when your receiving the eth it will automatically send your eth to creator/destinated address with forward function(Depends on the source code) i don't really understand how it work but maybe you can this source code
https://etherscan.io/address/0x1f0ed56f79b93c5fec57c0b5f29656c5e4b4c4d7#codeThen I go to work but as soon as the transaction arrives at the destination address I want it to be immediately exchanged for USDT.
Can I set this up on 1inch / uniswap?
It's possible but you need to make some bot or macro and integrate it with uniswap or any DEX API, i'm not developer though so i suppose you can search it on other section or search on some github repository, Good luck!