HPA tanks come with regulators already attached to control the output from the tank. They are typically either "high pressure (800-850) or low pressure (<400 I think).  You can put an additional regulator on, but they are not necessary. Some people like them, because you can tweek the output pressure of the tank to the lowest possible amount that will keep the marker working, thus increase efficiency and get more shots per tank. However.... since you have an A5 with a Cyclone that works off of blowback (excess gas/air), you need the marker to be somewhat inefficient as to continue creating the blowback. Especially if you plan on upgrading to a Response Trigger. E-grip won't be affected as much. A regulator can help fine tune the output, but it will take some fiddling with to get a good balance.  If you do go the regulator route, you may have to deviate somewhat from the instructions to get it to work properly. According to the ones that came with the Palmers Stab I had, it told you to adjust the FVA all the way out and then use the Stab to lower the output pressure to lower your FPS. I had to lower it so far that the cyclone quit working. This is where you will have to find the right balance between using the FVA and Reg/Stab to find the correct output pressure for your application. Personally, I didn't have the patience that day to mess with it and ended up just using the stabilizer on another project.

Low pressure kits have been discontinued, but you can still find them around. I don't have any direct experience with them, but what they do is lower the operating pressure that the marker takes to operate. I have seen a lot of posts saying they just aren't worth it.

Use the search feature to look up threads for the various topics you have asked about if you still have questions. Reading the stickies in each forum will provide a lot of info you asked about as well.