I use a custom 6.0.8 on my 4MB Plus and it flies.
I run Multifinder and have added the following to it:
-Backdrop (DA/INIT for putting MacPaint files on the desktop...that's right, dithered pics of my friends and I for the desktop on a Mac older than all us in the picture). This one is freeware but hard to find.
-SuperClock (cdev for placing a menu bar clock in the corner of the screen). Freeware, easy to locate.
-TrueType INIT, free from Apple's website.
-Init CDEV, an early Extensions Manager-type program. Postcardware.
I also have a few commercial products installed--SAM 3.5 (Anti-virus) and AfterDark 2.0. If you can't find the original disks for these on eBay or from one of the guys who sells old used Mac software such as B&R or Herb's, I recommend trying the Disinfectant INIT and Moire screen saver. Disinfectant is free, Moire is shareware.
And don't forget Bomb Shelter, the Andrew Welch INIT, to aid your way out of system error by forcing the continue/resume button to appear. Also, get Paul Mercer's Programmer's Key if you lost your physical key. It will work on a Plus if you use your ` key as the "power key".