Hello Sors, I see you've brought a lot of people with you.
It's hard to answer your question, but I will do what I can. Please don't be insulted by my answers, they are perceptions of what I know of Hermetics and Kabalah workings. I may make wrong assumptions, and I hope you will correct me on those, but I do have some framiliarity.
From what I've seen Omnimancy and almost any other magical system are compatiable to a point. The point is at the goals of why you practice. Omnimancy isn't a "spiritually fulfilling" system, nor is it supposed to be (although certain people have found it so for various non-standard reasons). Omnimancy is Thamaturgy, not Thergy. So people do magic and ritual mostly for religious/quasi-religous reasons. People practice Omnimancy for results. I think a good example of that is a student from a while ago who was a Thelmite (without being a Thelmite, long story), and he was amazed at the difference in abilities and efficiency. When I asked him how it compared, he said when he wanted to get more "fulfillment", he would still quite often do Thelmite rituals, however if he just wanted to "clean a room" for example, he wouldn't do a LBR anymore, he would just do a Z-rune from Omni and clean up the room in a few seconds with little effort versus how long it would have taken him to do the full LBR (let alone the time of operation).
The big problem most people face coming from Thergy is that Thergy is based partially on the concept that it's done that way because it HAS to be done that way. It's about belief. Omnimancy does things that "shouldn't be possible" but obviously it works and works well. So it's a matter of unlearning limitations that the Thergy system in question might have placed. Some people can't do it. No religious person has come through Omnimancy and hasn't been changed in some manner. Most have to choose between their old ways and drop Omni, or change their old ways. For some it's made their religeon stronger, and some dropped their religeon all together. It's mostly about why they were TRUELY religous in the first place.
A few have been able to resolve the the two. One of them has recently published a book of her journey's through magic, and some of the later chapters are on Omnimancy and how that blew open her world view and why. It's amusing to read because there was a lot left out, but I understand why. Here is a link to the thread:
http://www.omnimancy.com/phpbb2/viewtopic.php?t=624 While she is wiccan, the issues are relavant to anyone coming from Thergy. I have permission and plan to post the "Omni to Thergy" translator in the book (it was kinda a Q&A/FAQ). For her, her religeon became stronger, but did distinctly change.
Most of the problems is that Omnimancy is a lot about how things REALLY work, including astral interactions and modivations. Which can fly in the face of some over simplified religious points of view. Not that it really contradicts it, but it kind of de-mystifies it, and if the mystery is what you were really in magic for, it kinda kills that. Many people that I know that do ritual do it because it's fun to dress up, or the socialization, or the feeling of doing what they are doing. The fact that the spell itself might work or not is secondary and in some almost ilrelavant. That was the hardest thing for me to grip my mind around when Omnimancy started to expand since I couldn't understand that view point.
Omnimancy is compatible with other systems in where you upgrade different sections of the magical ritual. Meaning, you perceive the ritual more academicly/mechanically and tweak what is going on. In so doing, one can upgrade different parts to be more effecient, however, in most cases the most effecient thing to do is to do away with the ritual all together and just do it more directly.
Would you ask an angel/demon to do something for you if you can do it yourself as well or better? Would you do a 20 minute ritual to evoke or invoke a spirit you wished to talk to, or simply walk up and say hi and start talking? If you do ask an entity to do something, wouldn't it be useful to watch and be able to copy what they are doing, so you don't have to ask them to do it again for you, because now you can do it youself? And even when you do ask an entity to do something, does that mean they are going to give it their full strength, that likely you would have since it matters more to you then them?
I hope in my round about way I answered your question. Obviously, if not, please ask again what I didn't.
(And if you haven't read the Overview, it might answers some questions there.
http://www.omnimancy.com/OmnimancerOverview.html )
