15/7 time isn't too hard to understand. Basically you cut a whole note into 7 equal pieces and then put 15 of them into a measure, just like with 4/4, 6/8, &c. Of course standard music notation is best able to handle divisions of the whole note that are multiples of two.
Really, though, with a tracker you can completely disregard the second number in the time signature. It's only there for the standard notation with quavers and semiquavers and stuff. The important thing is how many 'pulses' there are in a measure, and the top number in the time signature tells that. For instance, if you track a piece in 12/8 time, it might as well be in 12/5 time or 12/100 time. It doesn't matter, although with standard notation you'd have to change the tempo to make a 12/8 and a 12/5 piece play at the same speed.
With a tracker, a better way to think about the time signature is called 'meter'. The meter of the song is (kind of) like the accent pattern in poetry. Duple meter moves in two, like
DA da DA da DA da DA da
and triple meter moves in three, like
DA da da DA da da DA da da DA da da
You could mix it up, like
DA da DA da da DA da DA da da
or
DA da da DA da DA da da DA da
or just use quintuple meter
DA da da da da DA da da da da
and so on, for whatever n-tuple meter you want. Most music is in straight up duple or triple meter, though, and occasionally switches from one to the other.
A nifty challenge would be to make two parts play in different meters at the same time...
By the way, onetwothree.xm by Loonie is a great example of a mod that plays around with meter. It's in triple meter, but sometimes it sounds like duple and triple played together:
ftp://ftp.modland.com/pub/modules/Fasttracker%202/Loonie/
