The two kinds of coffee art want opposite cups, and most people find this out with a wet piece of card in their hand. Free-pour designs are finished in the last few millimetres of liquid, so the cup is filled essentially to the top. A stencil is the reverse. No, you cannot use one in any cup: a stencil needs the drink to stop short of the rim, so the card can rest across the top without touching the surface. If the card wets, the powder clumps along the wet edge and the shape smears. The card is also ruined for the next cup.
Dhan Tamang, five times UK Latte Art Champion, is consistent about this across every stencil design in his book Coffee Art. The setup is always the same, and it has three parts.
Fill level. Stop pouring with clear space between the surface and the rim. The card sits on the rim, the drink sits below it, and the chocolate powder falls the short distance between them. That gap is the whole trick. It also means a stencilled drink is, by definition, a slightly smaller drink than the same cup poured for taste.
Foam. Tamang builds every stencil on a cappuccino poured with the jug held low and close, which produces a white disc of foam inside a darker ring. That is deliberately the opposite of free-pour technique, where you would start high to stretch the crema. Cappuccino-depth foam, around ten millimetres by Anette Moldvaer's definition, is dense enough for powder to sit on. A latte's five millimetres of wet, glossy foam lets the powder bleed and sink, and the edges go soft. His alternative canvas inverts the drink entirely: steamed milk into the cup first, espresso poured in after so it slips beneath the foam, leaving a clean white surface. It stencils cleanly. It is also a different drink, milk-first in the mouth, with no crema on top.
Rim. The card needs a level rim it can span and sit flat on. A straight, even rim of ordinary diameter works. A wide bowl the card cannot bridge does not, and a cup so small there is no room for a foam disc does not either.
What a stencil will not do: it will not hide coarse, bubbly foam, and it will not survive a cup filled to the brim. It decorates a surface you have already poured correctly, and that is all it does.
So the honest answer is that this is mostly a filling decision rather than a shopping problem, with one real exception: a shallow, flared cup filled properly for taste leaves you nowhere to put the headspace, while a slightly deeper cup with straight walls near the rim can serve both traditions. If you are weighing that trade, the wider argument that a cup's size and shape decide which drink you have actually made is where to start.