Venosa Violacea is a very good choice!
Yes, you can plant your new clematis to a 5 gal pot now. Brushwood plants have a very good root system and ready to go in the ground.
Do you plan to grow your Venosa in a 5 gal pot? Well, you can keep your clematis in a 5 gal pot for a couple of years, but If you think to grow it there permanently it won't be a good showy plant that you see on the pictures. It will survive but don't expect much from a rootbound plant. If you want it in a pot you need to get a 22 gal pot, at least 18 inches deep. Plus, from my experience, viticella clematises look much better in the ground. When mature, they have a massive root system which is water thirsty. Think how many times a day you will water your plant in your climate.
If you really want something for a 5 gal pot look for Raymond Evison clematises or other new varieties. Modern hybrids are more suitable for pots and usually have viticella genes to produce more flowers. I have Nubia and Chelsea. So far they look good in 5 gal pots but I am not sure for how long they can stay there.
