You can sell the portal and any associated guest race production buildings once you get into the next chapter. I do suggest building a bit of all the guest race goods first(they will live in your inventory even when the portal is gone). They are good for building roads and completing optional squad size upgrades that you have skipped (if any), if that ever becomes advisable.
Many chapters have a quest to get rid of that portal, some of which will award you diamonds. I am fairly certain the wording was made such that it counted even if you had already sold it, though, requiring only that you no longer have it placed in your city.
Short answer: yes. : )