Can I prune back and chop the height of a eucalyptus gunnii so it does not grow too high nor too wide? I wish to keep it in a very large pot. What type of soil would I need for the pot? Also is there any special things to do to a sarracenia hybrid in the

This particular eucalypt can be pruned to the ground every year in Spring, if that's what you want to do with it - it'll make a shrub about eight feet by 4 feet by the end of the summer if you do that.
The other thing you can do is to let the trunk get to the height you want and then keep it trimmed back to create a sort of lollipop - it does have very strong apical domination (means there'll be a leader shoot or three reaching for the sky) which means you need to keep an eye on it and prune those out regularly. It grows fast, so regular trimming is essential. It WON'T, though grow in a pot, no matter how large - it'll last a year or so if you're lucky and then die - this is a very deep rooting tree and if it can't put its roots down, it'll give up. Can you not find a spot in the garden for it - not near yours or anyone else's house?

And your query re sarracenia is incomplete - not sure what you're asking. They're tender and will tolerate down to 5 degrees C only, like partial shade and need to be kept wet except in winter and should be grown in peat and moss. And that's all I know about them;-)