Unless there were a lot of Droid commanders that countermanded the shut-down order, that's not going to mean much. Kalani and a handful of battle droids hardly qualifies as adding the Droid Army to the Rebellion.This REBELs show basically cemented that the clone army and the seperatist droid in the galactic area are on the same side and enemies of the empire.
The only way I can see it being feasible is if there are droid armies that were put into storage after being shut down instead of being scrapped, and Kalani reactivates those forces.
There is no Clone Army any more. There are three survivors that weren't rolled into the Stormtrooper Corps. As for the Droid Army, understanding the state of it now requires understanding the state of it back in the Clone Wars.
The Empire doesn't use a Clone Army or a Droid Army, they recruit and train soldiers rather than manufacturing them. The same is true of every state that follows.
Why?
The answer is that neither a droid army nor a clone army are the most effective means of creating soldiers.
The soldiers of a Clone Army are conditioned and trained since birth and given their similarity it's very easy to design training regimens that suit them all. Given how deeply they understand each other they also have very strong unit cohesion. When Rex says the GAR was one of the finest armies in galactic history he had a point.
A Clone Army however is phenomenally expensive and it takes ten years to get new soldiers. The time and resources that go into creating it could create a vastly larger army of recruited soldiers in a few months rather than ten years. While the Clone Troopers are individually probably the best soldiers in Star Wars one standard clone trooper doesn't equal twenty Stormtroopers.
The reason the Republic builds a Clone Army is because it can be manufactured in secret: a conventional military buildup would not go unnoticed by the separatist movement. The Empire has no such need for secrecy and does not continue to make clones after the Kaminoan soldiers run out. It's been solidly established that Stormtroopers aren't the equal of Clone Troopers but the Empire has far, far more of them and can replace them much more quickly.
But if the Empire values numbers why not use a droid army? While there's probably an argument made for droid soldiers being a symbol of CIS atrocities (hence the even worse treatment of droids in the Imperial era) a droid army isn't practical either. Droid soldiers are the opposite philosophy to clone soldiers: they can be manufactured at an incredible rate and come fully trained but they also have critical, exploitable flaws.
The most practical way of building war droids is to build large numbers of cheap ones: you press the advantage of it. If you're building a much smaller number of droids that are the equal of a highly trained biological soldier you might as well train the biological soldier: you'll get similar or better results for a lower expenditure. There's a reason the CIS doesn't mass produce the commando droids in lieu of B1s and B2s. Battle droids may individually be very poor soldiers but they can be produced in such overwhelming numbers that it's not a problem. A Clone Trooper may be the equal of a hundred battle droids but the CIS simply manufactures 200.
This is why the CIS thought their ultimate victory was inevitable: as long as their industry held they would eventually defeat the Republic by attrition.
The droid army only worked because the galaxy hadn't known war on this scale for centuries: a droid army this large was a new threat. The Republic adapted technologically to exploit the weaknesses in the droid army: relatively early in the war they'd already developed EMP "droid popper" grenades that could take out squads of them at once and as the war went on they got better and better at exploiting the droid army's flaws such as their lack of creativity and adaptability. You know how after a while you get used to a video game's AI and learn to exploit its flaws? The same is true of the droid army. Much like horse cavalry the dominance of the droid army was eroded by technological advancements it fundamentally couldn't deal with. This was hammered home when taking CIS high command allowed the total shutdown of the entire Separatist military machine.
It's possible to get old CIS gear including their war machines back online and many groups of pirates probably did. The galaxy is still littered with abandoned CIS equipment. There are probably pockets of droid army resistance that failed to shut down and don't know the war is over. However, they don't pose a threat to the Empire because the Empire knows how to fight them. They have superior soldiers, superior technology and now they even have superior numbers. Even if the entire remaining Droid Army came back online the Empire would probably win fairly easily.
If Kalani joined the Rebels then the best he could contribute is himself. Even then, he'd be of limited use: he's a Clone Wars era battle simulator with Clone Wars era information: he's accustomed to commanding battle droids. Whatever he's gone off to do it'll be something he thinks improves the Rebellion's chances.
You keep saying that only Rex, Wolffe, and Gregor escaped without the biochip, but we have no proof of that. A better thing to say would be, "So far, only 3 Clones we know of have escaped the Stormtrooper Corps".