I plan on using slip X thread bulkheads, with the threaded end under the aquarium, just because of the way my plumbing scheme works, if I were to glue the plumbing into the bottom of the bulkhead, I wouldn't be able to remove the bulkhead nut w/o cutting the plumbing.
My question is in regard to a spears gate valve on the overflow plumbing. What would be a better option, slip or thread? If I use a slip gave valve, I would put a union on each side.
Also, while I have your attention, if using threaded fittings do you guys prefer teflon tape or rectorseal teflon thread sealant?
Slip is always going to be a better option. Even with your bulkheads. There is only one reason you will need to remove the bulkheads, and that would be to take the tank apart for whatever reason, and a hacksaw will do the job handily, and will allow reuse of the bulkheads. You won't be removing the bulkheads with the tank running....
Why would you want unions around the valves? Good valves are serviceable insitu (without removal) so there really isn't a reason to remove them, unless tearing down the tank, and again a hacksaw works good, and the valves are reusable.
The point is threaded fittings and unions are just leaks waiting to happen. Unions are simply an abused item. You need two unions both at the pump, so you can get the pump out.
Tape or non-hardening thread sealant is not a matter of preference. It is made fairly clear by the fitting manufacturers that tape should not be used for plastic (PVC) threaded fittings. Tape is for iron pipe (and similar) to prevent thread galling. Such a condition does not occur with plastic fittings.
For pvc threads you need to use a non-hardening thread sealant, from Rectorseal, this is the one you want to use:
http://www.rectorseal.com/index.php/rectorseal-t-plus-2/
Spears also markets a thread sealant product that is suitable for the purpose.
Something labeled 'teflon thread sealant' may or may not be what you want.
Also, do not mix sch80 and sch40 threaded fittings. For further information on this topic, it can be found on the Lasco website.