Jay - NMF is a little different in the process...
First Alclad is good stuff, I just use Future over a well buffed fuselage, then spray the Alclad on top of that. Just use light coats. Actually instead of repeating a long paragraph go here: http://www.finescale.com/FSM/CS/forums/21/503355/ShowPost.aspx#503355 it's about in the iddle of the page... if you look through the group build most of the people used AlcladII on their kits.
I tried something I found while comparing reviews of the Tamiya P-47, Floquil "Old Silver" worked excellent as a Aluminum on my kit, it is about on the same level as Alclad as far as looks and a bit easier to spray.
I haven't heard much about Testors metallizer, I do know a sealant coat is needed on it or it can rub off. (They make their own brand sealer.)
As far as decalling you won't need a sealer coat UNLESS you use the Testors stuff, Alclad and Floquill are already slick and the decals will sit down good on them. the setting fluids don't disturb either paint either. After I did my last Alclad plane I shot a TINY bit of thinned future over the kit to overcoat the decals and highlight the shine in some places where light would hit it most. On the ones before I didn't overcoat at all and it was ok, the decals looked fine.
I wouldn't use a dull coat over metallics unless you were doing a specific bird you had pics of and could compare it to them. Dull coats tend to make the NMF look 'hazy' and whitens them a little. Just my take on it.
You should also be able to do any washes or what not without Future as the coat of paint is slick and the wash will not soak in.
To break it down:
If it's Alclad, sand with VERY fine grit paper, shoot with future till nice and glossy, let cure well before spraying.
Anything else, I would get the body buffed the same way to help the metallic shine better then shoot the paint. Once dried you are able to decal right away, wash and weather at will. If you want you can use future thinned well to give a bit of a sheen to vertain areas and help protect the decals (optional), perhaps Tamiya flat base in future at a very high future to base ratio (not dull it but take some of the shine off) for a more weathered plane.
Goos luck!!!! Either way you go you'll like the results. And if you use Alclad just remember light coats until it covers, it takes a few passes to get full coverage but you'll thank yourself for the effort and patience in the end.