The capacity of a derailer depends on the length of the cage and how it pivots. Suntour Cyclones of this model came in 3 cage lengths; RD-6000, 6200 and 6400, for short, medium and long, with rear cog capacity of 26, 28 and 30T respectively, according to Sutherland's Manual.
The capacity of a derailer installed also varies with the offset distance between axle and derailer hanger. You can enlarge that a bit by setting up your rear wheel farther forward in a near-horizontal dropout slot. These things are listed in manuals, but they're not etched in stone. Sometimes when the literature says 26, 28 works just fine.
I'm with John E on reducing the size of your chainrings if you want lower gears. Be honest with yourself about how high your gearing really needs to be -- there's always coasting!