Of course if the extra chairs you are using are pretty, it's perfectly fine to mix and match. In fact, that can add interest to the setting. Pretty wicker chairs from outside look right at home when brought inside. Using matching wingbacks or other upholstred chairs at the ends of the table add an elegant look. Even bringing in a bench from the entry or a loveseat from the living room works well.
And if they are very different? You may need to cover a bit more of the chair. Slipcovers will of course do that. While they can be a bit pricey (especially if they are custom), it's easy to make your own. Buy inexpensive fabric (painters' cloth from the hardware store looks great and cleans easily), cut large enough to cover the entire chair to the floor, then wrap the ribbon the same way. If you have rough edges on the fabric use fabric glue to hem them.
Whatever you do, remember it isn't about the chairs. It's about the joys of sharing your home with family and friends. And that's what they will remember.