I'll testify that the mere act of installing apps does not slow OS X down significantly the way it does in Windows. There really is no downside to experimenting with all the apps you would like to.
The first couple of years with my Mac, I installed and removed well over 200 apps a year. I have over 120 apps currently installed on this machine. My boot time after a clean install of Snow Leopard the first week after it was released was 28 seconds. My current boot time - 32 seconds.
For the most part, apps do not write anything into the system folders with OS X, so there is nothing added there to slow down the OS. You are now free to test out all the software you want, without the worry of it gunking up the gears of your OS.
The one thing to remember with removing the ones you test and decide you don't want to keep... if it had an installer, run it's uninstaller when you remove it. All of the ones (which are most of them) that are drag and drop into your Applications folder, you are good just dragging them to the trash. They leave nothing behind that is going to slow down your system.
My wife's Mac, now over 3 years old, has not had a clean install since it was brought home. It has gone through the upgrade process without a clean install from Tiger to Leopard to Snow Leopard. Hers still boots in just under 30 seconds and I can see no appreciable slow down. Show me a 3+ year old Windows machine that's been upgraded from XP to Vista to 7 that can boot in under 2-3 minutes and I'll give that man a cigar.
I'll also note here for the other new Mac owners reading this, I came to my first Mac as one of those hardcore hardware enthusiasts, building my own custom overclocked top of the line gaming rig once every year or so, reinstall Windows every 6 months, BlackViper was one of my favorite sites... You get the idea.
There really is no downside testing out all the software you would like. No more of that reading reviews to find the best one out there before you make a choice on which one you're going to get. It's freeware or shareware or has a free trial... download and install all you want.
Be happy... And free.