I got some small eucalylptus trees for that reason. I ordered leaves from that site, and got seeds too. However the seeds never came up. So I searched on the internet for some small-medium sized that I can keep inside because I live in Maryland and it gets cold and snows in the winter. Some of what I found shipped large and got larger still. This site turned out good small trees:
www.thegrowers-exchange.com/Eucalyptus_Silver_Drop_p/her-euc01.htm
The others I don't kknow about, citrus or acacia. I do know that I have seen posted on here that adding acacia to their diet helps prevent fur cracking during the winter, so I assume it is okay, but I would see what some of the more experienced people say. Sometimes bark is not okay.
The nice extra feature of the eucalyptus is the smell. The girls like to chew on the leaves sometimes when they wake up during the day (I put a leaf in their pouch). The smell is wonderful and sometimes I hear a crunching emanating from the pouch.