You may use a little Vaseline to help seal the bulk gasket if you like. Just a little bit around like you would on an oil filter for your car. As far as how to connect the flex tubing to the bulkhead, I would suggest a threaded bulkhead so you can put a threaded fitting in. This would allow for quick removal or adjustment if needed.
In my explination, the "long" end is the outside-threaded portion of the bulk and the "flat" end is the part of the bulk which looks like the top of a screw. Basically, it would go something like this:
Threaded bulkhead through glass tank port, long end inside, flat outside-->
Teflon tape on threaded fitting which will screw into flat side of bulk -->
Small PVC length inside fitting which extends 1.5 inches outside fitting -->
Slide flex tubing over PVC and clamp with a hose clamp.
In this scenario, the long portion of the bulkhead will be inside the tank/sump area and flat area of the bulk outside. Obviously, if you are using solvent (non-threaded) fittings, you cut out steps 2/3 and turn the bulkhead around. The long end of the bulk will be outside the tank/sump and the flat area inside. This will allow your PVC length to slide in and be glued directly to the bulk. The gasket always goes against the "flat" side
I put a thin bead of AGA silicone on the bulkhead side with out the gasket. Are you using flexible PVC? If so just use the recommended PVC cement. It will dry inert. if you are using clampable hose use a barbed fitting with 2 clamps and 3-4 layers of tape on your threaded joints.
How do I know what silicone is reefsafe. I know not to get anything that says mildew resistant. Yes I am using flex pvc. Like the idea of something that will not leak but be removable. The tank is currently setup with hard pvc but the sump isn't under the stand. I want to move it under the stand and flex pvc seems the best bet.