You'll probably find it impossible to read the in-camera light meter, the viewfinder being too dark to see it. If you are going to take a lot of shots over an extended period, the light meter might pay for itself in saved film. Yes, macro lenses are the best for this type of photography. It will handle specimens down to about an inch in size. But you still have to be careful to aim the lamp so the violet light isn't hitting the lens.
Some people use a black cloth drape to hide the lamp, hanging it to just outside the camera frame.
The aperture you use for any given shot depends on the size of the specimen you are shooting. An advantage of the macro zoom lens is that it lets you get in close, which is a plus when it comes to using wider apertures. And it helps to do so - cutting down on exposure times. Anyway, if you are shooting something relatively small and move in close with the camera you can get away with a wider aperture than when shooting something larger with the camera moved back. Start out by experimenting with your first few rolls.