Yes, well the Unitarians are a very weird group of people and I do not understand what they believe. I don't even think they know what they believe.
At least from what I've read about Unitarianism, it posits itself as simply a faith-affirming community, not a distinct faith unto itself. In other words, it's not really about doctrine, but of people from different religious backgrounds being able to lay aside their differences, worship together, and essentially believe that each other's faiths are equal and true paths to God. Contradictory beliefs about other faiths within the source traditions are then glossed over or written out for the sake of unity.
I may in fact be conflating this with Unitarian Universalism, but the similarities are strong enough I think it's at least partially warranted. So the end answer of what they believe is - 'only they know'. Or 'a lot of different things'.
Yes, as a Christian one should believe that God is the Creator of the universe. The only people I know who claim to be Christian and believe that god is not truly the creator are Mormons. They believe that mater has always existed independent of god. Of course they have a whole number of additional beliefs that do not jive with what it means to be a Christian.
Similar (not necessarily what Mormons explicitly believe, but similar) points also existed in the Early Church, due to many of the Classical Heresies. Gnostics and other groups often would consider the God of the Old Testament to be a malevolent Demiurge and that Jesus was sent by a different, higher God. The Demiurge part is where the Gnostic justification that physical matter is inherently evil and to attain salvation one must gain the knowledge to escape those bonds comes from. Then things get real interesting, as there is commonly a perception that this manifested in a couple different ways - asceticism to kill the flesh, which is what many feel Paul was referring to in his cautions against that, or hedonism borne out of the futility that as physical beings we can only escape it by actually dying, but since suicide isn't a proper option, might as well enjoy it...
This may have actually originated as hyperbole or propaganda rather than what ancient Gnostics actually practiced, but some modern Gnostic groups do hold more libertine beliefs than mainstream churches, with the use of 'Bride-Chamber' as a sacrament instead of Matrimony. The Ecclesia Gnostica, for example; not to be confused with the Ecclesia Gnostica Catholica that is aligned with Ordo Templi Orientis and Thelema, and is heavily sexualized - the latter is descended from Crowley, the former claims Valentinian and/or Rosicrucian heritage, which does actually put it into the category of 'Christian heresy' (or 'Gnostic Christianity', one of many in the overarching topic of Esoteric Christianity, tangentially related to the religion as Kabbalah is to Judaism - in fact, many Gnostic or esoteric sources draw heavily from Kabbalah).
For anyone wondering, that mostly comes from me having had a passing interest with it during High School, although it only ever amounted to an academical or sociological interest (any theological intrigue faded pretty quickly because some of it is just bizarre). The stuff is dense, very very dense - it did, however, inform me enough to be able to ID certain Gnostic-like* trends in the Modern Church, and precipitated my hard swing into the anti-dualist camp.
*for such fallacies like 'secular music is evil and inherently un/non/anti-Christian', which is where a lot of that influence is actually seen. The bubble subculture is another good parallel. In both cases, it's not explicit or even implicitly Gnostic, but it's the same danger of dividing the world and your environment into far too separate boxes - something the Gnostics
were guilty of.