I am so sorry you are going through this. I know how you're feeling, My boxer Joey was diagnosed with lymphoma right after Christmas, and my lab Katie was diagnosed with osteosarcoma about 3 weeks ago. Joey is on palliative treatment, and Katie's mass is on her sternum and is inoperable. Cancer sucks.
As far as pet insurance is concerned, I have a little experience dealing with some of the above mentioned companies. I have been a vet tech for 31 years.
Almost all of the pet insurance companies work on a reimbursement basis. You pay the vet, and they will reimburse whatever is allowable. That said, if your pet has had ANY kind of health problem in the past, it will be considered a preexisting condition, and will not be covered. This turns into a real problem, especially for allergy patients.
What a lot of people have started doing is opening a savings account for their pets. Every week, or pay period, they will deposit X dollars into the account, and use it for medical bills.
Care Credit is another option, but just be aware that if you carry a balance after the 0 % interest period (12 or 18 months I think), the interest rate shoots up to 24.9 %, which I think is outrageous. Also, you have to have impeccable credit to get approved...I know some people who have been approved for car loans that have been turned down by Care Credit.
Sending good thoughts for a positive outcome for your little Russell.