Was your "house" soil sample fairly close to the pond? Was the distribution of subsoil layers fairly uniform where you performed excavations at the house site? Does the current pond hold water very well?

I am asking these questions, because the biggest potential screw-up in your pond renovation/expansion plan would be piercing a laboriously constructed clay liner in the existing pond that was required to keep the pond from leaking.

I very much like that you had very stiff red and gray clay down to 18'. That PROBABLY means that they excavated the original pond in the shape they wanted, scarified (disked) the bottom and the sides, and then followed with light compaction to create a pond. IF that is the case, you should be able to do any work you want on the pond, and the worst you can do is excavate out some compacted material.

My advice would be to conduct another "soil test" when your pond contractor is on the site. If he has an excavator or backhoe, go close to the pond (but in an area that he thinks is probably undisturbed from the original job), and dig a quick test trench. Make sure you have good clay material down to 14' (if you want 12' of pond depth). Also make sure you don't hit any bedrock, or sand/gravel layers. If not, then your renovation should be a fairly simple excavation, then compaction job.

P.S. If you double the size of the pond, it will create a surprisingly large amount of spoils (dirt). You can do some rough calculations on simple volume math, but then you need to add 20-25% for "fluff". Dug material always increases in volume when you pile it on the ground.

Decide with your wife ahead of time what you are going to do with the spoils. Do not pay your contractor extra money to move dirt multiple times! One option is to create some nice berms with the clay as an attractive landscaping feature. You can then cover the berms with your stockpiled topsoil, and get whatever plantings you desire easily established.

Good luck on your pond improvements. Should be great fun developing your pond over the next few years!