If you're confident truing wheels it's fine to have different gauges on the pulling and pushing spokes, but it will affect the whole wheel not just the one side. It's OK to replace them one at a time and bring them up to approximately the right tension, but if you do that and roughly true after each spoke that's a lot of truing - I'd expect the rim to rotate slightly relative to the hub, and the dish will change too.
If you want an easy time then hold out for spokes of the same gauge as the originals, there will likely be a lot less work overall - after replacing each spoke the rest of the wheel will be pretty much unaffected.