You can use that fuel in your Rusty. Nitro % is just that, the ratio of how much nitro-methane to alcohol, additives and oil in that particular fuel blend.
You can use anything between 20 - 30%. The higher the percentage the cooler of plug you can run. 30% is generally gonna make your engine run a little hotter and its more expensive than 20% I generally use 20% for bashing and 30% for racing but you can use whatever percent you want just use the right plug and tune properly and it wont matter much. I use 20% for bashing just because its a little cheaper.
With a trx engine, you can keep the same hot plug that's in it now, even if you run 33% nitro. More nitro increases the possibilty of detonation, but there's plenty of forgiveness built into a trx engine. When you add nitro and/or detonation occurs, the best option is to add a head shim.
Medium plugs can be used in a trx engine, but they're typically for larger engines.