Seasoned DROID user, but new to the forum. Woot!
Anyways, here are a few things I will offer based on my own experience.
1. You only need taskiller if you don't want to reboot your phone frequently.
-I've seen too many people buy Droids and ask me the next day, "Why does it
run slow?" The answer I give them: "Because your phone is running EVERY
SINGLE app you have installed."
2. The Android OS is linux based. Linux is pretty secure. Not invulnerable, but pretty secure. It's pretty hard to get malware, viruses, adware, etc. on these phones. You would have to be TRYING to get these issues, and even in the case that you get infected, the OS is still pretty safe, though you would need to be worried about your information and data.
3.If you ever need to install an app from an unsupported source, make sure you disable unknown sources when you are done.
4.Most of the apps in the official android market should be secure and trusted by Google. This doesn't mean that all of them are of use. Yes, antivirus is a trusted app, but you don't need it, if anything, it's one of the last things you need.
I hope this info helps answer a couple questions.
Ciao! ^_^