In the US most septic tanks were (and many still are) cast from concrete. The inlet, outlet, and service cover are not sealed to prevent ground water from entering. If yours is concrete it probably wouldn't be good for washing clothes no matter how well you clean it. Waterborne bacteria makes your clothes stink. I'm dealing with that now.
As for balancing, you could put an overflow pipe with a check valve from the top of the new tank to the old tank. That way you could still use the septic tank for garden water. It would require more plumbing and a second pump though.
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