Magnification is always a tricky subject. A lot of people don't understand it or its limits.
As you already seem to know, the magnification factor is calculated by dividing the focal length of the telescope (in your case, 1,500 mm) by that of the eyepiece. In theory, then, you can magnify as much as you like to infinity, assuming you use shorter and shorter focal length eyepieces and/or extend the focal length using a Barlow lens.
But there are hard limits to magnification. When a wave of light enters the telescope, it begins to diffract. If you want to understand the underlying physics, there's a great Khan Academy video here you can watch to understand it better.
To summarize the problem: the light waves essentially break down and create interference patterns which start to degrade the image. This results in a limit of the information (detail) carried in the light entering the telescope tube. The larger the aperture, the more detail can be produced. While the exact process is different, the result is not unlike trying to blow up a picture. The more you try to blow it up, the blurrier it will get. I put together a photographic example here using an image I captured of the moon. Each frame in the sequence is effectively a doubling of magnification. Because imaging and visual astronomy aren't quite equivalents, I can't really give you a magnification power here, but you can see the net effect as I "zoom-in" on the crater Herschel.
In the mid 1800's, William R. Dawes devised a forumla for determining the angular separation required between two stars before one can reasonably state that they are, in fact, two stars. His formula provides a measurement in arcseconds we call the Dawes' limit that is frequently used as an indication of the resolution a telescope can provide. His formula divides 4.56 by the aperture diameter in inches to produce the limit in arcseconds (or 116 divided by the aperture in millimeters). For your 12" aperture, this yields a Dawes' Limit of about 0.38 arcseconds. Later in the 1800's, Lord Rayleigh devised another formula for determining resolution, which gives us a number, also in arcseconds, known as the Rayleigh Criterion. It is a bit more conservative, producing a lower-resolution limit. For your telescope, the Rayleigh Criterion is 0.45 arcseconds. A further calculation can be done to ascertain the theoretic limitations of the optical system based on the diffraction limit. For this telescope, that would be about 0.35 arcseconds.
Essentially what these numbers all tell us is how small a detail your telescope can resolve. This measure is, of course, an angular measurement as opposed to a linear measurement. The linear measurement depends on the object's distance. A detail that appears as 1 arcminute on the moon would be a few hundredths of an arsecond at Jupiter's distance.
Aside from the limitations imposed by diffraction/aperture, you also have the simple fact that the atmosphere limits your magnification. This is going to vary depending on your altitude (e.g. how much atmosphere is between you and the object), your light pollution levels, air pollutants, humidity, and air currents. For a telescope like yours, you can probably expect a maximum usable magnification of around 250x to 300x, depending on those factors. In fact, on some nights, 200x might be pushing it, while on others, 350x might not be particularly bad.
So, back to your eyepiece question.
As you've found, there are two common formats for eyepiece barrel flange size: 1.25" and 2". 2" eyepieces are typically only needed for long-focal length eyepieces for wide fields. In fact, for anything higher than about 25mm, it's entirely unnecessary. And I've never seen any true 2" eyepieces that have shorter focal lengths. The 2" compatible eyepieces I've seen with shorter focal lengths have always been combination 2" and 1.25", they accomplish this by using nested barrel flanges. The 1.25" flange is inside the 2" and slightly longer, so that it will fit into a 1.25" focuser recepticle. Here is an example. Notice how the 1.25" flange protrudes below the 2"? This allows you to use it in either size focuser.
So, what should you get?
This is partly a matter of personal preference. I use a set of Meade 5000 Series eyepieces (the earlier versions with the 60° AFOV). In that set, my shortest focal length is a 5.5m which on my 10" Dob (1200mm focal length) gives me about 220x. For the seeing conditions where I am outside of Houston, that's usually about as good as I can expect. I have other eyepieces I occasionally try out. I have a 3.6mm (not sure who made it), which gives mea round 333x. It's rarely worth using - it just gives me a bigger blur. Something around 5.5 or 6mm is about as high as is worth going.
If you want to do it on the cheap, the Orion Expanse series isn't too bad, and you can find an un-branded version online cheaper. The Expanse 6 mm has a blue line around the barrel toward the eyepiece. If you do a search on sites like Amazon you can find the same eyepiece with a gold line instead of the blue for a lower price. It's a dirty little secret in the telescope market that the majority of consumer telescopes and accessories are manufactured by only a handful of factories in China and re-branded for sale. I have a Meade f/6.3 focal reducer and a Celestron f/6.3 reducer... they're identical other thant he labelling (and I've seen the same thing sold by Antares). The Orion SkyLine 8" Dob is manufactured by GSO out of China and the exact same scope has been sold as the Apertura AD8 and Zhumell X8 (and by a few other brands as well). In this case, the only difference in the eyepiece between the Orion Expanse and the gold line versions is the name and color of the line.
Finally, the question of "to Barlow, or not to Barlow." Barlow lenses put more glass in the optical train, this is true. But these days, that's not a huge problem, as long as you get a fairly good quality Barlow. Generally speaking, however, it's better to go without than to use one, if you can avoid it.
So, overall, you're not really getting anything out of a 2" eyepiece that you can't get out of a 1.25" when you're talking higher end eyepieces. For proof of that, look at the TeleVue line. TeleVue arguably makes some of the best, if not THE best, eyepieces on the market. Their high magnification eyepieces are either 1.25" or dual format. If there was a reason to make the high magnification eyepieces in true 2" format, they'd definitely do so. So there's no loss in going with the 1.25" and saving the money.
