Great question! In order for a reel to properly fit on a rod it needs to do two main things:

  1. It must have enought capacity to hold the fly line and backing with a little room to spare.
  2. It must balance on the rod so the rod is neither tip nor butt heavy. 

A Lamson Speedster reel for a 5/6 is usually the Speedster 2. It is designed to hold a 5 or 6 weight line and is not large enough to hold an 8 weight line and 150-200 yards of backing.

Second, and in my opinion most important, is the balance issue. The Speedster 2 is so light that the rod would be tip heavy. You can check this by putting the reel on your 8 weight rod, holding your index finger straight out horizontally, and placing the rod & reel on your finger. I properly balanced outfit should balance somewhere near the front of the cork. 

My suggestion is to get a larger reel to use with your 8 weight rod and keep the 5/6 reel for your lighter rod. Thanks!