Is the SolaX X3-FIT W X3-FIT-6.0-W compatible with the SolaX DTSU666 + 1000A CT?

Yes — listed by the manufacturer.

The manufacturer's published pairing data lists these together.

Manufacturer's compatibility listListed together in the manufacturer's published pairing data. A cited document reference is being added as datasheets are processed.

Pairing details

CT specification
cts: 1x grid CT (+ optional CT2 for AC-coupled PV) · note: For longer runs, use M3-40 meter via RS485 (up to 1000m). M3-40-Dual monitors grid + 3rd party inverter simultaneously · type: Split-core, RJ45 connection · cableLength: 1m · maxExtension: 100m (standard RJ45 coupler + network cable)

Both products

Design a system with this pair