It would have to be a 30 amp breaker to use the full power of the welder. I'd say go with the 30A. The general rule is that your planned load should only be 80% of the circuit capacity. That means a 30A circuit should have a maximum load of of (30*0.8) = 24A. With the 30 amp breaker you must have at least #10 wires feeding the circuit.
if i have a breaker that has a 120/240v and my dryer has a 240v plug can i change the receptacle to a 240v
The voltage is correct, BUT, Your welder is going to draw more current than the pool pump did. The breaker, wiring, and the receptacle will have to be upgraded. In my opinion I have never seen a "temporary" connection!! ...pkazsr
For a 7000watts or 7kw a 240V then it is 30 Amp
Yes, because you will be able to maintain current flow between phases.
I suspect you mean GFIC breaker. The dryer will not cause the breaker to fail.
Simple answer is yes. But you need to first determine if your breaker can handle it, not just because you have an empty slot. You have to total up amps being used and check rating for your box. If it can handle it then it is a sinch to install