S1 Pro and S2 Pro do not need and if I understand correctly do not support DTTL.
The following is extracted from a review of S2 Pro (which uses TTL flash in a similar way to S1 Pro) from http://www.bythom.com/fujis2.htm Flash sync works to 1/125 of second. Flash metering uses five TTL sensors and can be balanced, slow-synced, and synchronized with the rear curtain. Unlike the Nikon digital SLRs, Fuji seems to have made Nikon's regular TTL system work just fine with the SuperCCD. In other words, the Fuji S2 can read TTL values by reflections off the CCD during exposure. The drawback to this is that TTL is only supported to ISO 400 (I suspect this has something to do with the changes Fuji made to the TTL meter reflection readings, as the N80 body supports higher ISO TTL). Thus, all your old Nikon Speedlights (e.g., SB-24, SB-25, SB-26, SB-28) work just fine with the S2--you don't need a DX-type Speedlight. Indeed, it's a joy to be able to use a wireless SB-26 with TTL on the S2, something that simply blows away the Nikon digital flash system.
1. Probably. The DX is meant for Nikon DSLR cameras.
2. SB26 is more powerful and has a wireless mode but is bigger. Performancewise, it's definitely better than the SB50DX. It's already end of line though, so you could only get them in the 2nd hand market.