Honestly, I did not have to fuse two or more toys together for this build [which I often do]. But in that case, always make sure that your toys are roughly the same height, before you try to swap limbs [because that will mean that they're roughly in the same class; like the Legion class, Leader class, Deluxe class, Voyagers etc]. And unfortunately, all the TF toys made over the past 20 years do not all have compatible joints, so you may try to fuse & kitbash a guy with "clicky sounding" ratchet-joint legs with a guy that has fluid balljoint legs, and that can be a hassle unless you know how to unscrew / cut the ratchet-joints open carefully...
Plus, large balljoints can be sanded down, so that they work better with the limbs that you are attaching. And putty can be smoothly added to a balljoint's head, to make it larger, if necessary.
Also, I added two arm-mounted weapons from one of the Iron Factory third-party toys, as well as two more laser handguns from the Combiner Wars Leader Class Skywarp toy [that toy comes with like six weapons]. Thus, in some pics, my custom toy has two weapons; while in others, he has four. And the former Transformer toy had skinnier legs void of the outer wheels, and no doors attached at his back.
Anyway, check out the front view, side view, back view and top views of the new Turbo Blitz:
LASTLY, MY BRIEF TOY MODEL CUSTOMIZING AND PAINTING TIPS:
Honestly, I did not utilize a base paint coat or any primer for this job...
The thing is, those base coats that lie under the paint can be either good or bad, depending on what you want to do.
One of the reasons why people apply base coats, is to mask an underlying layer of color that you don't like at all. So if you intend to paint with a bright surface color, then you should use a white / bright base coat. But if you want to eventually paint your toy with a dark surface color, then you should utilize a dark / black base coat. That way, if you miss painting any edges, then the underlying darkness or brightness will easily hide the former color, causing the toy to look more balanced and somewhat "properly battle-worn".
So if you utilize a paint spray as the base coat / primer, you need to let it dry for maybe 36 hours or more; and then you should get a surface paint linked to the brand of the primer that you used, if possible. But even then, if your new paint is too "thick", then due to the base coat you applied first, the eventual color of your toy model will look lumpy and the underlying details of the toy will be fully overshadowed by the surface paint + the base primer's color tone.
To reduce the thickness of the final surface paint that is used as your top-coat, you should use a thinner... perhaps even rubbing alcohol, which you can buy for less $2 at a grocery store... or if you choose to buy a professional thinner from a prominent Japanese brand like Tamiya, they will sell you something really small for a large price.
You need to mix that thinner into your paint, by adding a few drops of it - since you're basically diluting your paint and making it flow better. These are actually hand-painting tips, which is my preferred style. And those who use the mechanical airbrushes don't have to worry about using thinners as much, unless they specifically want a thin color layer as their top-coat.
Note: Always let paint fry for over 24 hours at least, before you start fiddling with the toy.
And as far as paint type goes, if you are a humble beginner, go with sharpie markers or acrylic paints [from a brand like Deco Art or Tamiya] for your toy models. Try out the paints on some cheap / used toys from ebay, to get a feel for what you are doing. When you've been doing it for several years and you've learned advanced blending tricks by trial and error, then you should switch to enamel paints.... I've heard that they are brighter and they last longer, but they require special base coats, or else they will melt / weaken your plastic toy. But I believe that the Enamels are better on metal toys.
And remember, Part 1 of this article with more pic & details on customizing Transformers is over HERE!