I've always machined aluminum with normal drill bits and tooling, whether it's castings, tubing, weird alloys, you name it. Nothing special in my machining experience, and I get beautiful chips. Use some cutting oil and change your feeds/speeds if you're shredding it. I like the way Tap Magic Aluminum works, but I hate the way it smells, especially when I smoke it up, parting things on the lathe on a taper. WD40 or 10W40 or even Pro Link wet chain lube or Fluid Film is also fine. Basically anything is better than nothing for most things. Then stainless and inconel often do better without cutting fluid as long as you don't let your tooling dwell and your workpiece harden, but I digress.
The derailleur will take up any out-of-round you introduce, and it's a granny gear, it's not like you're going to spend a ton of time in it. I would just go for it. I would rest the chainring bolt flats of the crank on some blocks to make sure it's parallel with the face of the crank. Depending on your drill press table, you may be able to stick an appropriate sized bolt up through the table and put that through the square tapered hole so that you can put all the holes the same distance from the center. Using the chainring as a template is also a good idea. I'd bolt it down to the first hole you drill/tap and then each consecutive hole. Do them in a cross pattern, probably. If the surface you're entering is at an angle, it may benefit you to buy a 1/4" or 5/16" flat endmill to throw in your drill press chuck and touch to the face you're drilling to make it flat so that the drill doesn't try to move as it's entering an angled face. Plunging an endmill should not be a problem for your drill press chuck, despite what others may tell you.
Oh, and you know about the trick to use a
tap guide in the chuck of your drill press to align the tap and provide a little pressure, right? It requires your work to be held down to your drill press table, but it makes tapping a lot easier. Just don't move anything after you have drilled the hole, chuck the tap guide in your drill press, and pilot the tap with it.
Now that I have access to the tools, yeah, I'd program a mill to CNC drill it. Because I can. But if you have a good eye, you'll be surprised how close you can get with normal tools.