HI,
You're mixing up a few different plugins.
ServerRewards value from BotSpawn uses the ServerRewards plugin.
Economics is a different plugin although the next version of BotSpawn will support that in the same way.
I'll ensure that both accept a floating point value so you can give 0.* rewards.
RustRewards is a totally separate thing and gives out its own ServerRewards, Economics, or Scrap.
It has its own values and multipliers, all adjustable through its config, and it does support BotSpawn, although it treats all BotSpawn npcs equally.
The advantage to giving out the rewards via BotSpawn is that you can do it per-profile so the next update of BotSpawn is probably the best solution for you.