I tried this idea with mixed results. First, the "aquarium" tubing seemed too small to accept the caps. Then I tried two kinds of commercial plastic caps, blue and black. It wasn't too difficult to push the caps into either type using the plastic tool provided.
I used these to cap OEM nipples on a Uberti 1861 Navy Colt using CCI caps. The blue types seemed to be a little bigger than the black type and tended to bind the cylinder when it was rotated.
I had no problems firing the gun. Sometimes the plastic cap cover remained intact, sometimes it did not. Sometimes it stayed on the nipple, sometimes it did not. I tried removing the spent cap from the plastic sleeve and found that difficult to do and gave up on the idea of reusing the plastic caps. I used both #10 and #11 caps since the OEM nipples seem finicky on which fits best.
I've ordered some Slixshot replacement nipples and Remington #10 caps and will see if that has better results.