A WAD manager can only brick your system when you're installing channels with broken banners or things you're not supposed to install (like a stub system menu IOS). SM64 Multiplayer is totally safe, I use it myself
I'd recommend the english translation of the german translation of YAWMM (lol) here:
http://www.hacksden.com/showthread.php/7241-YAWMM-DE-English-Translation - It's currently the best WAD manager out there.