Hi and welcome!
Personally, If you want to work on a line of combtails/CTDT I would Go for it, However The only problem I have is with your male. His color and fins are kind of all over the place. I would find a better male, possibly a blue or maybe a green DT that is more balanced and has cleaner cut fins. Here is a website that can give you more information on the DT
To answer your questions,
1) They are both betta splendens, aka same species. I think you meant tail type
F1 generation (your babies from this spawn) will produce a mess, if you were to sell them to a mom and pop pet shop as OldFishLady Said, they would probably take the fry (baby bettas) But most people like nice colors and finnage, as you can tell most of the bettas that sell quick in a pet shop are the ones with big gorgeous fins and eye popping colors. Anyways, The tails will do well later on in the line, however genetics isnt ever that simple as any science major like me will tell you
There is always a deviation from what is expected.
2) The male determines when he builds the nest! Some do it when they see females, some do it once she is released, and some when spawning.
The key thing is to have your supplies ready for your spawn
Here is a very helpful page With great information
I think before you start breeding make sure you have this down like a science. I am starting my own line of halfmoons and I have been doing my research/ obtaining supplies. And it can be cheap, since I find my stuff on sale or it can be very costly. I like thrifting it helps!
Personally if you have homes for the fry, and you have a long term goal in mind, Research and start a spawn log!
One reason people are so "offended" is because The tail types you are crossing will mean you have to do MASSIVE culling to get the intended combtail or CTDT without passing on "bad genes" or what they like to call "pet shop betta genes" since most fish at the local petco or petsmart are mass bred and are a complete mess genetics wise. And most first time breeders are not prepared for doing mass culling. Basically you have to euthanize any fry (baby bettas) that show deformities. I would personally use the clove oil and vodka method, or the freezing cold water method. Some have told me they feed the fry to other fish, but if that is too jarring for you I would research other methods. There is alot to breeding, Alot of research helps. Please do not feel offended as many of us just want to do right by the fish and too many times on here people throw a male and a female in the tank before they know what to do and end up killing fish and it really gets frustrating. Study keeping the species and hold off your plans for a bit and It will be much more rewarding in the end. Sorry if I come off as rude, I don't mean to be at all!
Personally once I start breeding I'd like to try and produce a line of CTDT/Combtails. They are GORGEOUS! but I need to know more about genetics and the inbreeding process and how to do things correctly.