For T shirts, I'd use a 70 or 80 ballpoint, depending. If you don't want waves, then be very careful not to stretch the fabric while sewing, let the machine feed it. I use a walking foot on top to help and a longer stitch. And don't be afraid to steam iron a little after, just be careful.

If you mean slits, not vents, you have to open the seam the length of the slit and open the hem a little on either side. Put the hem back first and then fold the seam allowances back in on either side. Stitch up one side of the vent, straight across the top, and down the other. How close you stitch depends on how much seam allowance you have, try to catch most of it so it won't roll in and show.

Twin needle I only tried once, and got a ridge like a pintuck, so I'm thinking the bobbin tension needed loosening. Actually, I was about to post the question myself, so hopefully someone else will be able to help both of us.

Thanks so much. This is extremely helpful.

Using a straight stretch-stitch helps with not having the fabric stretch while sewing, but it takes a while(two stitches forward, one back, lol).

Turtlelady explained making slits nicely.

I've never tried twin needles, but I've done two rows of stitching 1/4" apart on t-shirt hems(I only turn the edge up once), then trim close to the edge stitch. It seems to help keep the hem from rolling up after laundering. Is that what twin needle stitching is supposed to do? Been sewing a long time but mostly by the seat of my pants :D