nope... unless you have a specific instruction like a Service letter or kit that says otherwise, the weight of the item is the weight of the item. the Spec lists weights. if in doubt, look at the weights of other like items in the Specification like propellers or tail wheels & see if that makes sense to you. and, you don't have any payload to spare anyway, the weight of the gear has to come off.... subtract the weight of the gear & add the weight of the floats. the wheel weights are in the Spec, but the gear legs and cabane, etc aren't listed, so you will need to weigh them. as for the weight of the floats listed in the Spec... that would be as they came from the factory, so any extra hatches, pump outs, or spray rails would not be accounted for. also prolly came with only one water rudder.... food for thought. you can consider the arm of the gear to be the same as the wheel. that and the arm of the floats are in the Spec.
because you have a nonstandard arrangement (C90) you will need to do fore and aft adverse load checks in your W&B as well as a basic.