Are you spraying the High Shine colors or the regular colors? If its the regular shine colors then you don't need any special under coat. You can spray it right over the Tamiya primer. Just make it as smooth as possible to get the best results with your Alclad II.
If you are using the high shine colors then you will definitely want some kind of gloss black base. I recommend using Alclad's own Gloss Black Base. It works really well and dries in minutes. Whatever problems you may have heard about in the past have long since been fixed.
BTW, you can seal the Alclad II paints with their Aqua Gloss...even the high shine colors...and mask over them.