I have an AAC TiRant 9S.  I want to be able to shoot it on my MP5K.  Need a tri-lug adaptor and I’m just curious if I will still need to utilize the fixed barrel spacer with the tri-lug adaptor?  Thanks!

You do not. I have a MP5 clone along with an old school TiRant 9 and I don't own the fixed barrel spacer. If you had the spacer, the spring loaded portion of the 3 lug Triad would not compress and allow the can to mount on the gun. The spacer is for using one of the pistons on a non-tilting barrel.