If you want to do DAA or GGG, it's best to get more dulcimers and string them with special strings--folks mostly use banjo or guitar single strings in specific gauges. But yes, for DAA, you'd tune the top string down to unison with the middle string, and it's serviceable with standard strings. With tunings like GGG there are two options: all strings in unison ("Galax" tuning) or the melody and middle strings an octave above the bass ("bagpipe" tuning)--I wouldn't try either with standard strings. Unless you're singing, you can shift the key to accommodate the limits of your strings and play the tab or arrangement the same way.

The 6-1/2 fret, if your dulcimer has it, makes it possible to play in keys that would otherwise be outside the range of your strings (either too high or too low), particularly if you use a "reverse" tuning: tuning the middle string to the tonic of your key and the bass string a fourth below, to the fifth of the key.

I can't give you hard info on string tolerances, but as a general rule of thumb, I don't attempt to tune higher than 3 or 4 semitones above the standard tuning. Tuning down, the only limit is how floppy the string becomes. Given the same gauge and type of strings, the upper limit is also determined by the scale length, which varies substantially with makers and models. So if your scale is longer relative to the norm, the strings will break at lower pitches.

The Friends of the Mountain Dulcimer site (currently in transition) is the best one I've found, but you can also post your questions on Everything Dulcimer ( http://www.everythingdulcimer.com/discuss/index.php ).

I posted a rather lengthy message in another thread here dealing with tunings, capos and "false nuts" that may be of interest:
http://mudcat.org/thread.cfm?threadid=19254
That message is in the resurrected part of the thread, where the individual messages are all glommed together, but you can search for my handle (of course, the rest of the thread is worth a read, too). My message starts out with comments on peg dope, doubled strings and railroad spikes before getting to the good stuff. Also check out the related thread links in the top part of that thread.