Yes, most things should work on one or two legs of 120/208Y. 240V heating appliances will draw a bit less power. Motors may draw a bit more current for the same load (or maybe at less than full load will have better power factor?) What wouldn't work is a 120/240V autotransformer or two 120V transformer primary windings with neutral also connected.
L6-30, 3 wire ground/hot/neutral (or ground/hot/hot), 30A 250V
Does your generator also have 120V outlets? If so you should be able to install a 4-wire split-phase outlet.
The 3-wire outlet with ground should not be used for neutral and two hots, even if the source has a grounded centertap, because your loads could get electrified ground.
If it only has 240V and no center tap, a transformer could be used to produce either split phase or full wattage at 120V.
If you were to use the system to backfeed into the grid (or backfeed into grid connected main panel, but with a zero-export configuration using current transformer to sense), there is a 56A limit per Sunny Island (that's the 6700W limit, at 120V). Perhaps there is some way to have a device control Sunny Boy's power output while grid feeds 60 Hz through Sunny Island to Sunny Boy; I think they do that at least with some newer models.
If you tell Sunny Island not to backfeed the grid, it can treat grid same as it would a generator - connect when it wants power, disconnect when PV output exceeds loads and battery charge needs so nothing backfeeds. In that case you can have 10kW of Sunny Boy per SI-5048US or 12kW of Sunny Boy per SI-6048US.
About the only difference between the two is 6048 seems to be a bit more efficient, can output 5750W continuous at 25 degrees C while 5048 can output 5000W. Both have same surge capability. 6048 has larger battery terminal, while 5048 has smaller double terminal.
With 2x SI (either model) wired 120/240V split-phase, you could have 1x 10000TL-US-12 (240V model) backfeeding grid, or 2x 10000TL-US-12 not backfeeding grid.
With 3x SI (either model) wired 120/208Y 3-phase, you could have 3 x 6700W = 20,100W, 2x 10000TL-US-10 or 10000TL-US-12 backfeeding a 3-phase grid. (I think; they have to rearrange watts between legs to do that.) With 3x inverters not over 6700W each, could do the same.
If you set up a 120/208Y 3-phase system and feed it 120V single phase, you would be limited to 6700W backfeed. I haven't figured out yet if 4x SI can be wired 120/208Y with two in parallel for 13,400W to/from grid.
If you use the grid only as a generator, don't backfeed, then you can have 3x 10000TL-US (either model because both do 208V) for 30kW of GT PV on your 120/208Y island grid.
Oh, the 10000TL-US has a firmware bug that grid-backup mode doesn't curtail production. SMA investigated my inquiry and told me to set it to off-grid while using it on grid behind Sunny Island, with the Sunny Boy backfeeding the grid. I suggest you do not do that unless they give you such instructions in writing; it differs from what documentation says to do. I used it that way for a while then replaced with 5000US. I felt there could be a safety issue that SMA Germany foresaw but SMA support in U.S. didn't know about.
Two 120V hots 120 degrees apart in phase is 208V. With just one Sunny Boy across L1 and L2, nothing on L3, it drives current with a phase shift. Same with two Sunny Boy. Three Sunny Boy distributed across all three pairs, and they add up to in-phase current on each line.
On SMA's site, I'm only finding a newer document without older models listed. Download and save the following:
I just got my 3-phase setup running. 3x SI-5048US wired 120/208Y, old tired 100Ah 48V battery bank (4x 12V batteries), 30A dryer cord feeding master SI, 3x auto-transformer 120/277V (actually a bit low, 120/260V) making 277/408Y (actually 260/450Y), one Sunny TriPower 30000TL. Just one 2500W PV string so far, about 380Vmp.
It does transfer power on all three phases and exports to grid over the one connected 120V 30A line. When I disconnect grid with battery full, SI raises frequency to 60.5Hz and TriPower disconnects. I haven't connected a data cable yet. It has SpeedWire (Ethernet) installed, and I've ordered a RS-485. The TriPower is supposed to support UL-1741-SA and off-grid. No parameters whatsoever in the manual, will have to see what is there when I get it talking.
One weirdness is my Harbor Freight clamp meter shows current in the 120V legs, e.g. 0.50A with sun low and clouds. But on the 277V legs it shows zero. Using DC setting I do see current in the PV wire.