Decimals when adding addresses

Hi, how do you know what decimals to put in when adding a new address? I have just bought some refined and not showing up when added it and used 1 for decimals but not sure what to put… Thank you

If I want to buy more of the coins on my Trust Wallet am I better to do so with USDT or ETH?
Which would incur less fees?
I have about 18 coins I want to add funds?
Thanks in advance