My 2-cents-worth.. I'll give you some specifics, and some general tips as well..
1. Do I need primer for flat enamel paints or just when using gloss enamel or lacquer paints?
You don't need primer for either, although it's ok to use, especially over dark-colored plastic that is going to get painted with a lighter color.. I use the grey automotive primer from Wal-Mart... It's dirt cheap, works really well, wet-sands beautifully, and you get some minor sand-scratch filling from it...
2. Is the order: flat paint, then glossy clear coat lacquer, then decals, then flat clear coat laquer?
If you paint in gloss colors from the beginning, you can eliminate the clear gloss step. But generally speaking, no... My order of finishing is:
1- Color coat
2- Clear gloss (generally-speaking, acrylic over enamel, enamel or laquer over acrylic)
3- Decals
4- Gloss coat to seal decals.
4b-Fade base colors (optional, and separate technique to learn, which is beyond the scope of this reply)
5- Wash. For oil and enamel washes, this is why you use acrylic clear. The thinners will attack the enamel color coat if you don't.
6- Clear flat (Again, acrylic over enamel, enamel over laquer over acrylic so the next step doesn't attack the washes and finish)
7- Drybrush highlights. Use enamel over acrylic, yadda-yadda...
8-pastels (optional, but almost required for exhaust and gun-powder stains, and applying dust) Pastels require a flat finish to work best because the rough texture gives some "tooth" for the powder to grab..
Note: Don't over-do the 'dust-color' pastels... If you do, your model will look like it taxied throgh a flour-mill..
Note 2: Forget about using Future if you don't have an airbrush. Personally, I only use it for what it's designed for. Floors. I mean, I tried once, just because everyone raved about it back in the 80s... Didn't like the result, it was hard to control, and finicky about thinning... Rattle-cans of clear gloss & flats take about 10 seconds to apply, dry much faster, and there's ZERO clean-up.. If you DO have an airbrush, you can probably use it... Personally, I hate the stuff, and if I didn't, I'd never suggest it be used by a beginner. Rattle-cans are far better (IMHO) for beginners. Crawl, Walk, Run...
3. Is weather wash the very last thing to do or does it need to be clear coated?
No to the first. A wash is tinted thinner (not thinned paint) that's usually applied to give depth to recessed areas. The only "weathering" that washes are good for is for POL stains Petroleum, Oil & Lubricants) like fuel, oil, hydraulic fluid-type stains, drips, & runs.. It works best over gloss, which allows for capillary-action to carry the wash into the recesses and lines. For most POL stains and seepage, it has the best effect over flats. Takes practice though. A single pin-point drop can bloom into a pretty big stain... Clear coat it when you're done, but remeber.. When you add depth, you'll also need to add hight. So after you clear coat the wash, you'll need to drybrush the highlights, usually in progressively lighter shades of the base color. Drybrushing itself is another, separate technique that's beyond the scope of this post. Rather than trying to get all your questions answered in one post, I suggst you do some more reading on painting & weathering... I HIGHLY recommend the Bible.. That is, my Bible; How to Build Dioramas by Shepard Paine.
4. Should gloss enamel ever be clear coated?
Personally, I clear-coat EVERYTHING, gloss or not.. Clear-coating after you've applied decals, washes, and drybrushing tie everything together and make for a uniform finish and the decals look painted, the washes look like shadows, and the drybrushed highlights like reflected light, which is what you want..
5. Should I have bought the metalizer paints instead of regular enamel metal?
Metalizers are a different animal than metalic enamels. They require an entirely different approach to painting... They MUST be applied by airbrushing unless you use the rattle cans, may or may not be buffed, and MUST be sealed with their own type of sealer. On the other hand, if you want to use them, the rattle cans are easily applied, but still require sealer. What I'd recommend, and what I do more often than not, is look in arts & crafts for metalics...
There's two products I use extensively, one being a rattle can of Krylon Premium Sterling Silver metalic for Polished Aluminum. This needs to be applied over gloss black for best results. It can then be masked in various places and shot with clear flat or gloss to give different panels different shades. Use it in conjunction with other Model Master sprays. It doesn't require buffing to bring out it's shine and is shiny enough to reflect a fingertip... The other I use is Silver Rub & Buff... It's a paste that comes in a tube and you apply it with a Q-tip, brush, or finger... It takes parcatice, so don't use it without practicing on a paint hulk and on several different undercoat colors.
All that said, and given that you're doing a Strike Eagle, most of this won't matter since you're using that dark grey more than anything... I imagine that the most you're using the metalic for is for the 'burner-cans and such areas of the tail-feathers.. I'd use Testor's Model Master Steel for that area, and brush it on, with a wash made up of 10 parts thinner and 1 part burnt umber or black, then drybrush with Aluminum... You might have to add a bit more color to the wash at 10-1, but use that as a starting point. You can always add more color or add more layers, but it's hard to take it out... Remember that your wash will dry lighter than it appears when it goes on too, by about a third to half..
Claer as mud, right?