I use nylon washers quite frequently. If it is something that is going to be heavily loaded and might compress the nylon washers then I would use a thin Delrin spacer.
Whatever sort of spacer you use, take into consideration the loading so that you use a material that will not distort under the compression
. Teflon is pretty soft but under light loading works great. Nylon is a little harder and Delrin is very hard and very resistant to distortion. Definitely use the Tef-Gel for the stainless steel
fasteners into the aluminum
I find that with caulk for this kind of application it almost always fails because it eventually delaminates from whatever it was applied to, allowing water
I have successfully stopped Awlgrip from bubbling up from aluminum
corrosion where I have threaded stainless into aluminum by bedding the heck out if it with Tef-Gel. You may not need any plastic spacer at all depending on the application. Tef-Gel will remain a soft sticky gel that displaces water for many many years without changing its physical properties or washing
away...that's the beauty of it.