Yes, 1620 addition and subtraction do stop if the addend is shorter than the augend and there's no further carry.
There was no pre-defined limit to the length of integer operands. If you had the memory to do it, you could add two 10,000-digit operands. Floating-point numbers were limited to 99 digit mantissas, however. On 1620 Monitor I, the default FANDK option was 4-digit integers and 8-digit floating-point mantissas.
