Make borrowed chairs work around one table
Different chairs can share a table comfortably when seat height and movement come before appearance.

Borrowing two chairs for dinner solves a seating shortage, but it can introduce a different problem: one guest ends up perched much higher than everyone else. Before thinking about how the chairs look together, compare how they work with the table. A chair that feels comfortable alone may not fit beneath its apron.
Place each borrowed chair where it will be used and sit down. Check the room for knees, the height of the seat relative to the tabletop and whether arms prevent the chair from coming close enough. Avoid trying to correct an unsuitable chair with a loose stack of cushions. A better chair is preferable to a clever-looking workaround.
Walk around the table with the chairs occupied or pulled out to a realistic position. A wide chair at the end may block the route to the kitchen even if its legs fit neatly beneath the tabletop. Different backs and legs can also overlap when chairs are pushed in, so test that position as well.
Repeat a small visual detail
Once the seats work, connect them through the table setting. The same napkin color at each place draws attention to the shared meal. A simple cloth can soften the contrast between an upholstered chair and a wooden one. You do not need slipcovers or matching ribbons to explain why the chairs differ.
If there are two similar borrowed chairs, placing them at opposite ends can make the arrangement feel deliberate, provided both positions have enough space. Do not force symmetry when it produces an awkward seat. A comfortable asymmetry is better than a balanced photograph with one person trapped against the wall.
Check every chair for damage and stability before use. A decorative stool or occasional chair may not suit a long meal or the intended user. Borrowing from a familiar room does not remove the need to inspect it, particularly if it has been stored for a while.
At the end of the evening, return the chairs with any agreed care in mind. If the same shortage occurs frequently, note which borrowed chair worked best. Its useful dimensions and shape give you a starting point for a future purchase, far more informative than simply shopping for a chair that resembles the existing set.