I see the Force as just another name behind the energies that assist in both Left Hand Path and Right Hand Path magick, Seeker. As such, my particular brand of Sithism has much in common with occult traditions and beliefs. Performing a ritual, meditation, prayer, spellwork – it’s all using the Force, even if by a different name.
That being said, I do believe that the philosophy and practice of Sithism is compatible with other beliefs. One can be a Satanist and a Sith, a Druid and a Sith, any flavor of modern neo-paganism and a Sith, a Thelemite and a Sith. Sith philosophy does conflict quite a bit with the philosophies of some major religions, such as Christianity, though I would argue in practice that it does not. You’ll have a hard time finding a Christian who is truly a Christian. However, more than practical behavior, the fundamental exclusivity found in revealed religions will exclude a belief in the Force – even though prayer is an active use of the Force.
Other major religions, such as Hinduism, Taoism, and indigenous traditions have no incompatibility with Sithism – at least to my knowledge. Many traditions have the description or belief of an all-encompassing or omnipresent force (with a small f) that, even if they don’t describe it as the highest power as I do, does not contradict Sith philosophy.
This is a short one, Seeker, but the only point I wish to get across to you is that most people use the Force, whether they call it by that name or not. Magickal workings call upon the Force, and a mastery of the Force will lead to a more powerful working. Go forth, Seeker, and may the Force ever serve you.