I have a few carling style switches to mount for some new accessories. Any tips for cutting these rectangular holes in the helm?

I would go with a RotoZip type tool. Make sure you use a TILE bit, not wood.
You have to move fast enough that you don't heat up and snap the bit, yet slow enough to keep control. You should be fine on such small cuts though moving slow, but smooth.

Mask over the whole area, and make your cuts across the masking tape. If you can have a second pair of hands (helper) there with you, have them running the shop vacuum right next to you to keep dust and debris down. You might even want them on the backside of the dash if there is access.

Do your plunge cut in what will be the waste portion. Do this about 1/2" in from each corner, and move out to the exact corners, first. That way you will not over shoot when doing the sides/top/bottom.

There are some threads on here with same advice for cutting speaker openings.

WEAR GOGGLES! (all parties)
IF one of those rotary bits comes apart, pieces go everywhere.
Good luck.

A Dremel is another great tool to have. As said previous, mask the area and layout your holes. I would use a small (1/8" or smaller) drill in the corners then us the Dremel with a 1/8" carbide bit to rough the holes. Then use a file to finish fit the switch.

I like to drill the corners also with a regular drill. Also be careful of the Dremel bit "walking" across your dash. They spin very fast. Put down a towel for the dust to fall on. The dust is light and if you use something hard it will just blow off it. Have a shopvac handy. Make sure there is nothing behind where you are cutting so you don't cut anything you don't want to plus there is enough room for your install. No worse feeling cutting something out and find that what you wanted to install wont fit because of an obstruction, at the wrong angle, wont wire up, antenna wont work, etc, etc.... and you are left with a hole to fill.

I'm not a rotozip guy, but I do have these amongst other items :