May I make the note that the 650c idea was popular back in the early 2000s, now no tri bikes from any of the major manufacturers use 650c wheels except for in their small sizes. (at least none that I can think of)
Also, If you have a 700c or even 27 inch wheels, you can't put 650c wheels on your bike. The bike needs to be made for 650c wheels. (or if you REALLY wanted you can but it's not really worth all of the extra drop bolts and long reach breaks you would need)
Some people have preferences on what angle to mount bottles, I have heard straight up keeps the bottles in more, but if you have good cages and you don't go over too many big bumps, it won't matter. Whatever angle is there now should work.