I have successfully installed PV disconnects in addition to the six existing 'utility' service disconnects twice. In one case our PV disconnect was the seventh disconnect and in another we had two 800A PV disconnects in addition to the six 'utility' disconnects. Each service 'source' is allowed to have six disconnects according to the code, utility, wind, solar, cogen, etc. I would recommend confirming that your AHJ is up to date on this allowance AND will allow it before proceeding though as many have their own requirements and/or interpretation of 230.2(A), 230.71(A) and 230.82. 230.71(A) being the most critical because it allows up to six disconnects PER service and 230.2 defines the utility and PV as two separate services which would therefore allow up to 12 disconnects.
You can not rely on the inverters required anti-islanding features to shut down the PV service supply when it is tapped on the line side of the utility disconnects as most all commercial systems should be.
Most of the PV disconnects we install are in the 600A to 2000A range and are subject to the 10-25 foot tap rules, BUT some AHJ's and inspectors have asked why we're installing the disconnects so close to the utility service since we had inverter disconnects 200-800 feet away, they are actually considering the feeder a utility lateral and as such do not see a need for the additional fusible disconnect since the utility is not required to do it. We install them regardless of that though as the fire department is usually more interested in having the disconnects as close together as possible.
Even AHJ's that want the PV disconnects within 10-25 feet have allowed us some leeway when there was NO way to get a 1600A-2000A visible blade fusible bolt switch that close, as close as possible was still acceptable even if 50-75 feet was the best we could do. We have not done this, but they have been free with expressing their allowance of this.
