Primer and paint serve different functions... IMHO the primer/paint combo products are pure crap-- they're for occasional use by people too lazy to actually do a PROPER paint job... for lawnmowers, yard furniture, and other such things where the actual finish appearance doesn't matter, they're probably okay... and if it makes it easier for those type users, appealing to them as a marketing ploy, so much the better as far as the paint companies are concerned.
For the best finish, you need to apply a few coats of primer, let the dry, and then sand, usually with a 220 grit paper to take the surface down and level it out, then a 400 grit to really smooth the surface out. You might have to repeat this step a time or two to get the surface really smooth and blemish free... Personally I even sand the surface with 400 grit paper that's been dipped in water and then lightly daubed off on a towel-- this "damp sanding" really smooths the surface out very, very smooth. Basically, the surface should be as smooth and blemish-free as possible after priming and sanding as you can possibly get it... sand only in small circles to avoid/minimize sanding scratches, and always keep the paper moving, so you don't sand in flat spots... Basically, the surface finish you have when you finish sanding the primer is as good as it's going to get... Hold the rocket up and observe the reflected glint of light off the tube from a bright window or lamp or light across the room-- if you can see any imperfections in the reflection, you want to fix them now, or decide if you want to live with them.
Remember, paint does not cover up imperfections, it MAGNIFIES them... so immaculate surface prep is a MUST for an excellent final finish. While "doing the primer finish right" is essential to a great finish, you can STILL mess the finish up if you don't apply the paint right. Multiple thin, even coats are what you want... Runs, drips, or sags will ruin the finish and set you back to having to sand off the affected areas. The final coat of paint you'll apply a little heavier, so that the paint flows out smoothly and evenly onto the surface, without "orange peel" or dry spray (speckles of paint particles that don't flow out into the paint film on the surface).
Anyway, that's the basics... all this has been covered any number of times on the forum in numerous threads... but as for your original question, NO, I would NOT recommend the combination paint/primer products... Find a paint system (primers/paints/clears/etc) that you like and that works well for you, and stick with it is my best advice... mixing and matching different products is asking for problems as well IMHO... it can be done, but you HAVE to know what you're doing... and do paint tests to make sure that it will work... and then, in some cases, some prayer never hurts...
Krylon used to be the 'go to' brand for rocket finishing, now IMHO their products are pretty much absolute crap... I've used the stuff, but it's ALWAYS my paint of last resort-- only for colors that I can't get in any other brand... it's finicky and even when applied following the can instructions and wait times TO THE LETTER can STILL go wonky and ruin a paint job halfway through the process... or worse yet, at the VERY END... IMHO there's MUCH better product brands out there that work extremely well without all the hassle, some of them are cheaper and everything else is about on par for cost... Personally I've had EXCELLENT luck with the WalMart Colorplace primers and paints... the only bad thing about them is an EXTREMELY limited color palette... but for plain black/white paint jobs like a lot of scale NASA type rockets, they work very very well. Rustoleum has some good paints, and Duplicolor is top of the line... Testors works very well but its expensive due to the small can sizes.
The main thing is, practice, practice, practice, and know what you're using... the different formulations don't always play nicely with each other... for instance, you can apply enamels over lacquers, but not the other way around (99% of the time-- some folks DO, but it is NOT a beginner technique...) Most paints on the market are alkyd enamels, but the different manufacturer's tweak their formulations enough now that you have to be careful mixing brands on the same paint job... for instance, applying Rustoleum over Krylon might not be a problem-- OR, it might "alligator up" and wrinkle all to h3ll on you within minutes of spraying it on, ruining both the paint job and the primer work you've already done. That's where paint tests come in.
Best of luck! OL JR