I am using Densguard for my back board, and red guard for the seams and to cover the nails. What morter should I use for 6X6 porcelin tiles? I don't want anything fast drying (am a slow worker when it comes to installing tile)

You do mean DensShield tile backer I hope?
If it is DensShield, then a latex modified thinset mortar is what you should be using. If your slow just mix smaller batches of thinset or use a Type 1 mastic.
The same thinset or mastic is used to embed the Alkali-resistant mesh tape for all the joints.

If you are going to use RedGard waterproofing, then you will have to use thinset for the taped seams.

DensShield specs would prefer you to caulk all abutting panels with silicone while assembling.

I can't be specific since you didn't register where you're located or what brands you have access to. Generally you need to use a modified thin set mortar. It'll say "modified", latex, or polymer etc.

The spec it meets is A.118.4 and A.118.11 If it says only A.118.1, you're got the wrong stuff. You must use modified to install porcelain, (with the exception of going over Ditra or Kerdi) and Dens products tell you to use modified regardless of the type of tiles you're using.

Jaz

I forgot to mention, this is for tub and shower walls, but not for the shower pan.

My reply was based on a tub/shower enclosure. There would be no change in specs.