I'd go with the Corsican mint......creeping thymes would really prefer to be in full sun and a northern siting with minimal winter sunlight is not going to be their cup of tea :-)

Ask at your local nursery/garden center for the availability of growers flats. These are solid (no pots) flats of groundcovers that are typically used by landscapers, although helpful for homeowners with large areas to cover. They tend to be about 1/2 the cost of potted plants. With low, mat-like, root-as-they-go groundcovers, you can just separate appropriately sized plugs and plant in between the pavers. Each plug will have enough roots for the plants to 'take'. And you can space the plugs to fill in according to the spacing of the flags and your needs, not by some prescribed spacing that may not apply to your situation.

Groundcover seeds are not easy to find but that would be an equally inexpensive approach although much slower.

How small can I cut them, I think a flat is like 20" x 20" or so.

Nuursey flats come in different sizes...square and rectangular. Here, most common are rectangular ones approximately 11" x 21" or somewhere in the neighborhood of 220 square inches. Plugs can be as small as an inch square, so you can theoretically get as many as 220 (or so) starts from each flat. If you got the larger, square flats (about 17" x 17", or about 275 sq. in.), you would obviously get a larger number of starts or plugs.

Make sure you are asking for growers flats, not plug flats. Growers flats are groundcovers grown in a sheet, making cuts of whatever size you want easy. Plug flats have the groundcovers already growing in tiny individual sections. They can work well also but might be more expensive than the 'flat' flats and with less flexibility on size.

For SF Bay Area, is this the right time to plant them since cold rainy season is coming? Thx

Grass?