i have a torker unistar LX and i want to know what the difference between that and a nimbus 20" with isis hub is. are the cranks shorter is the seat lower i mean really what is the difference
The nimbus uni is a lot stronger in general and will take a lot more abuse.
You’re Torker has a square taper hub, as opposed to an isis hub. If you do tricks that put a lot of torque on the wheel, or like jumps and drops, the torker won’t last as long. Also, the torker has a weak wheel with a single-walled rim that is more prone to bending and whatnot.
The cranks are shorter if you want them to be. Crank length depends on what you want to do, but in general, longer=more control over the wheel’s acceleration while shorter=faster/smoother.
if the cranks were longer and im really small would i still be able to reach the pedals while at the bottom of the revolution
on a 20" unicycle unless you are like 4ft tall you can use any sized cranks. i would recommend 4.5-5"
he im 4foot 7 so im really short for a 12 year old
oh so at least you will grow. before cutting anything, i would consider buying like a 16" or 18" because theyre cheap.
Cut the seatpost. If you happen to eventually outgrow it, a new seatpost is wayyyy cheaper than a 16-18 inch unicycle. Plus, any tricks you learn on a smaller unicycle you’ll have to relearn on the bigger one, plus they’ll be much harder.
Actually im not gonna grow that much taller and it will take about 6 years for me to grow to full height which will be like 5ft 6 or 5ft 7 
Shorter means a lower center of gravity - so maybe you should try to look at that as an advantage! 
I’m 6’4", so it’s a long way for me to fall, and I also have a really hard time buying shirts. . . .
Back on topic, stick with the 20", and upgrade to the Nimbus when you can.