Then the load break rating would be the max power current of one string and the interrupt rating would be the sum of short circuit currents of all the combined strings - so the interrupt rating would be a few times larger than the load break rating. Correct?
Correct. It could be 20 times larger, or more, if you have a large combiner. It's probably a good idea to use the short-circuit current for one string for the load break rating, because under ideal conditions the max power current may be more than nameplate. This is not likely to require a higher rated device in most cases, I would think.
I just noticed something that may provide an explicit answer to my original question. in 2011 NEC, 690.16(B) requires "The disconnecting means shall... comply with 690.17." 690.17 requires "The disconnecting means... shall consist of a manually operable switch(es) or circuit breaker(s)" So that does not allow a touch safe fuse holder or pullout regardless of its rating, does it?
I don't know, can a touch safe fuse holder be considered a switch? It meets all the other requirements if you can consider it a switch.
Also note the exception to 690.17. I think that means that if you have a load-break rated disconnect on the output side of a combiner, you can use connectors right outside the combiner (i.e. within sight of and accessible to it) as the disconnecting means on the source side. The connectors are not load break rated which means your servicing method is to disconnect the whole combiner on the output side before disconnecting source circuits. You can do this even on a system that is running and has other combiners. This is a pretty cheap and easy solution, and something I recommend in any case to help commission and service systems. I suppose theoretically if you don't want to work in the combiner 'hot' then you disconnect
all the strings before working in the combiner. But if the work is just to replace a fuse in a touchsafe fuse holder... At any rate, I believe such a setup meets the letter of the code.