In OpenMPT, you can even change the highlighting of the rows to make it easier.
You can write stuff in just about any time signature you want that way.
As to triplets -- well, they're a slightly different kettle of fish. Say your crotchet (quarter note) lasts four rows. That means that two quavers (eighth notes) fit nicely (two rows each). Three quavers, however, don't fit as nicely -- they take 1.333 rows each. There are a few ways that people use to get triplets, though:
1. Use the note delay effect (in IT, SDx). This isn't that precise, but it gets pretty close. It's the method I use and recommend, especially if you work in nice large groups (personally, I always have eight rows per crotchet.)
2. Change the tempo (or speed) for a while, then change it back. In other words, increase the speed by 133% for the duration of your triplet, which will 'look' like normal quavers, and then change it back afterwards. This has the drawback that everything in the pattern will be sped up.
3. What most people seem to do is to use dotted notes, rather than triplets. In other words, instead of having three quavers as a triplet, they have two dotted semiquavers and a semiquaver. In terms of rows, this means that in eight rows of crotchet, notes would be on rows 1, 4, and 7 (indexing from 1).
Hopefully I haven't expressed myself too unclearly...
Let me know if I have.