How big is your Jardini? If it's under 12-14", I would wait until it matures to add more tankmates. Most Jardini seem to be mildly aggressive when smaller, and then snap once they are a foot long or a little bigger. Your Jar may be "not so bad" right now but change completely and not tolerate anyone. Or, maybe you got lucky and got a particularly mild Jar. But, no way to know until it's full grown. They grow pretty quickly as well, so having patience until it's an adult shouldn't take all that long.
The other factor to think about is that if you DO wait, your Jar may get used to living in the 300g alone and not take kindly to newcomers. So that's another consideration. But tankmates with a Jardini are always a gamble, comes with the territory.
What I would do is this; wait until the Jar is about 14". See how it behaves with the tankmates it has. If it's killing them, you've got a solo fish. If it's still some mild nipping, maybe you've got a chance. I would change up the decor in the 300 and try to add 2-3 fish at once. Big catfish, gars, that sort of thing is what people usually try.
