Welcome to the club Brad! And happy holidays to you and yours as well.

Let me see if I can help with a few of your questions.

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Here's the deal with the voltage regulator,  aka VR, tubes. The tubes have an actual voltage drop as follows:

0A3 -> -75V
0B3 -> -90V
0C3 -> -105V
0D3 -> -150V

It's been a while since I looked at the manual and I don't think the 0B3 is mentioned, but they are a thing. Regardless, using a 0A3 would drop the B+ voltage 75V, which is stock, and by stock I think what Steve means is what is hardwired into all the other UFO type amps. Using a 0D3 would drop it 150V, or 75V below stock. Using a 0B3 or 0C3 would drop it in between.

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Firstly, everyone is different. Different tastes, different systems, different rooms blah blah blah. So rule #1 is do what sounds good to you, not what "most people" do. One of the joys of owning tube gear is experimenting as much (or as little) as you want. For me, I typically like the 0C3 tube in front but the only way you're going to know is to try them all. And don't forget the bias switch at the front of the amp, it also affects the sound. It kinda splits the difference between adjacent VR tubes. But it also affects the volume so make sure you adjust it, otherwise the louder setting with usually sound "better."

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Different tube manufacturers (or the same manufacturer at different times) made tubes that look different but have the same electrical properties. Vacuum tubes were the "high tech" of the mid-20th century and tube companies were constantly trying to find ways of making the same (or equivalent) tube cheaper.

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It's up to you. Tube clipping is a gradual thing and a slight wiggle means you are just getting there. If you don't hear anything wrong with the sound then there is nothing wrong.

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Indeed. It is a remarkable piece of gear and I expect you will enjoy the heck out of it. If you haven't already I'd encourage you to spend some time reading the various fora, probably starting with the UFO25 and Replacement Tube fora to get a glimpse of other's experience. There is a great deal of wisdom on this site.

Welcome BradC81 and congrats on the new amp.
No doubt you're in for a great experience.

I think it's a great idea to use the pre-out of the integrated. As good as the UFO25 sounds on its own, as long as the impedance is matched well between the two, the ability to use the multiple inputs and connect your sub should enhance your experience. I find using my CSP3 helps make the UFO25 all it can be.

I'm relatively new to tubes and the Decware sound as well, with a little over a year under my belt. (Although I was browsing the forum for years before I got off my butt and finally ordered something) When I got the amp I wondered like you did, how to start off with what tubes. I decided to put the OA3 in and use the 7DJ8 as a starting point. I put about 30 hours on them and then changed out the OA3 for the OC3 (leaving the 7DJ8 in). I put another 30 hours or so on them before switching again to the OD3. Then, I put in the 6N5P to replace the 7DJ8 and started rolling the VR tubes again. So again, about 30 hours each VR. I wanted to get some meaningful hours on them so I could really evaluate the combinations once the amp had seasoned in. Obviously, the sound changed a lot not only due to the different combo's but the seasoning of the amp. Once in a while I would also flip the bias switch to get a sense of what changed. Since I have higher impedance speakers (16 Ohm) I found early on that using the 'hi' (pushed to the rear) setting worked best.

Once I had over 250 hours on the amp, I then went through all the combinations again with a critical ear at that point. I tried to really evaluate what each combination brought to the table. It was a long process, but I thought the only way to really understand the differences. Since every system and room are different, we will probably all settle at a different configuration. In my case, I was using the OD3 and the 6N5P. To my ears, that was the smoothest and most enjoyable combination. Once I had over 1200 hours on it, I got some NOS tubes to further refine the sound (and to use the stock tubes for backups if the need ever arose). Bottom line is that whether you take a shot gun, or a methodical approach to tube rolling, you'll figure out what you like so enjoy the ride!