Yes, being restricted to specific finder framelines is a traditional limitation of rangefinder cameras that have combined range/viewfinders with multiple framelines, and it's basically unavoidable because the finder magnification is fixed (with a few exceptions such as several Canon film rangefinder models that had switchable magnifications.)
It's a tradeoff: higher finder magnification gives you more focusing accuracy (because you can see the coincidence of the rangefinder patch more easily) but also limits the wideness of lenses the finder will accommodate.
There have been a few attempts to market a modern rangefinder camera specifically for wide-angle lenses, the most serious one being Cosina's Bessa R4a (auto exposure) and R4m (metered manual exposure.) These were Moses-format (36x24mm) film cameras, which had a viewfinder that would take in a horizontal width of 80 degrees; this allowed them to provide a finder frame that covered lenses with a marked focal length of 21mm (along others for 74°/25mm, 66°/28mm, 54°/35mm, and 40°/50mm.)
This was handy for wide-angle lens enthusiasts, but it limited the camera's flexibility because the finder's magnification was only 0.52x (per the
chart on the Head Bartender's website) so if you also wanted to use longer-focal-length lenses, you also needed to buy a Bessa R3a or R3m, which was exactly the same (excellent) camera with a beautiful 1.0x-magnification viewfinder and framelines for lenses with 49°/40mm, 40°/50mm, 27°/75mm, and 23°/90mm.)
At the time this wasn't a popular solution, and Cosina was phasing out its Bessa film cameras anyway, so the R3/R4 twins didn't enjoy a long production life... with the result that you'll now pay a couple of grand in US $ on eBay for a clean example of either model. But if you like to shoot film with a rangefinder camera and your favorite lenses are wide angles, it's still worth the splurge because it spares you the need to track down auxiliary viewfinders.
Most RF cameras adopt an intermediate finder magnification in the interest of covering a more useful range from moderately wide to moderate tele, or else fudge on the visibility of the widest frames. For example, the Epson R-D 1 -- which shared the rangefinder optics and basic chassis of the Bessa R3a -- had the R3a's beautiful 1.0x viewfinder and a Super 35-size sensor of 25x17mm. This combination allowed it to cover a useful range of view angles: a semi-wide 48° (obtained with a lens having a marked focal length of 28mm), a normal 40° (with a lens marked 35mm) and a mild portrait/tele 28° (with a lens marked 50mm.) The kicker: The normal and portrait framelines were fine, but the semi-wide frame was so far out toward the edges of the viewfinder that it was just barely visible to the naked eye, and eyeglasses wearers basically had to imagine where it was.
The Pixii makes a different and arguably more useful compromise. It has the same Super 35/25x17 sensor size as the Epson, and covers the same 48°, 40°, and 28° view angles (plus throwing in a longish-normal 35° frame that services lenses with a marked focal length of 40mm.)
But the Pixii drops the finder magnification down from the R-D1's 1.0x to a much smaller 0.67x. That means images seen through the finder are significantly smaller -- but the semi-wide frameline is easy to see even with glasses, making it much more usable.
This sensible compromise does mean that if you want to take in a wider view than 48° on the Pixii, you'll need to use an auxiliary viewfinder. Back when the Epson R-D1 was new, Cosina made several wide-angle viewfinders specifically for its 25x17 format, so if you can locate one of these you can simply buy the one that matches the marked focal length of the lens you want to use.
If you want to use an auxiliary viewfinder marked for the Moses format, you'll need to do some simple arithmetic. Multiply the marked focal length of your Pixii lens by 1.53 to find out what Moses-format viewfinder you need to look for: for example, your 103°/10mm lens will need a Moses-format viewfinder marked "15mm" and so forth. Reminder: You are NOT "calculating equivalent focal lengths" or any such nonsense -- 10mm is 10mm is 10mm. What you're doing is the same kind of conversion you'd need to do if you have a thermometer marked in degrees Fahrenheit and you need degrees Celsius -- you're just converting from the marks you've got to the marks you need.
