Can I use opcodes traditionally used for verifying keys to verify payment sizes?
This isn't possible. 

You can use those arithmetic operators to compare any numerical values that you can get onto the stack - they are not inherently restricted to signatures and keys. But you would have to get the relevant data (output addresses, output amounts) onto the stack in the first place, and there is no opcode  to do that.   The script doesn't get to see that information, so you can't  restrict the spending of a transaction in such ways.

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