Does valve stem diameter matter for my rims?
Yes, the holes are different sizes. 6mm and 8mm, from memory. The hole size matters if you have skinny rims as 8mm out of a 20mm rim is a lot compared to 8mm out of a 40mm rim. But you already have valve holes.

In theory putting a presta valve in a Schrader-size hole matters, in practice it doesn't.

The base of the valve is where you'll see any effect from having a presta valve in an oversize hole, and in theory the base of the valve will get cut up by the edge of the rim. You might also get dust or water inside the rim if you have a V or box rim.

In practice it doesn't seem to matter. I do this fairly often because almost all my rims are drilled for schrader but I'll fit presta tubes rather than ride on flat tyres). I've seen marking around the base of the valve when I've pulled the tube out, and occasionally it's bulged up 2-3mm.

If you look at the locking ring/nut that comes with Schwalbe presta valves it's got a smooth face under the knurled section that's exactly the right size for a schrader hole, suggesting that they expect people to use them that way.

 

The ring I'm talking about has an arrow in this pic, and @jimrings found the pic on the right.

There are a couple of other types of adapters. You can see the most common ones in the pic below.



The ones to the right and left push through the rim from the inside and hold the valve centered in the rim. The rubber one on the right actually attaches to the rim. The metal one on the left is fitted over the valve before inserting it into the rim.