Seresto has only a single product for cats in all weights. You can use Seresto for Cats for your Mainecoon.

1) To prevent your dog from missing out on any treatment, we set it as auto delivery every 6 months 2) There is no expiration date on the Seresto collars because the product has been proven to have a long shelf life. It has been tested for a 5 year shelf life as long as it is unopened. Once the product package is opened, the shelf life is 8 months.

Yes, you can use them together.

You can use this product safely any cat who is 10 weeks of age or older. It is long adjustable collar so weight won't be a issue.

Yes this product effective on sizes. You just need to adjust the size.