The 25 amp cable will be fine.
Assuming you want to keep your cut out switch(and you do) just take the live output after the cutout SW, to the relay.
Doesn't matter where the earth comes from but just take it from a ground, or the earth wire direct from battery that feeds pump earth.
The realy should be wired as this,
30 Live direct from battery(via a 25amp fuse)
85 Earth
87 Live output from relay to pump live
86 Live from/(after) cutout switch, make sure it is after or just use the original live that went to the pump.
I really would recommend a seperate new earth, but up to you, as otherwise you still leave a 20+ year old wire and connection on one side of the pump and earth is just as important as live, some take from a point inside the car alternatively though, which is still far better than the original wire.
All the ecu does for the pump is output a LIVE to the cutout switch then from there to the pump, the pump earth does not come from ecu, but from main loom earth points.
tabetha