Welcome! I'll go straight ahead to critiquing.
Concept wise, I think the idea that two entities acting as one is pretty cool, something unlike the ones we normally see such as Dugtrio and Magneton. The Tag gimmick you gave them are also fairly unique, giving them more reason to utilize their co-op system.
However, beyond the concept, your design (as in the elements making up your art, not your art itself), doesn't give off the "feeling" of a Pokemon. Since it's easier to break it down, I'll list the things I see as things to improve on:
- The changes between all 3 stages don't seem to do much. This gives it the problem fakemon designers are usually known for, which is the "it just gets bigger" syndrome. I'd recommend making each stage look independent, while connected. This is easier if you make the first stage have less features, and then as it evolves some parts become changed while some new parts are added. This ensures that you're never just adding onto the design, or just changing it to a completely different one either.
- Despite being part Steel-type, I can't really pinpoint anything on their designs that helps me tell they're part Steel. The only Pokemon that I believe does this is Lucario, so unless you can explain it using Lucario's logic, I think you should bring out the Steel type in the design and make it more prominent. For suggestions, you can also add the Steel-typing in later evolutions. This could be useful with the first bullet point I listed, since it's another way you can add differences to your evolution stages.
- Considering the body shape, I think it looks sort of generic. Since they look sort of like foxes, why not play around with shapes related to foxes? Like maybe more of an animalistic set of legs, or maybe give some definition to their arms. Usually, making them look simple is fine for smaller species, such as Meowstic, but once you have a bigger design the simplicity looks fairly awkward, as if they're just a giant doll-shaped monster.
In short, try utilizing more interesting shapes. You've already have a grasp on pattern design, (which is awesome by the way!), so I hope you'll at least consider the points I have above for your next endeavor.