In my experience, most of the "can be trained as a climber" comments apply to warmer climates than your zone 6a. Most Austins can get huge in places like California so those kinds of comments are a warning to people without cold winters to indicate this might be an "octopus arm" Austin.

I grow all of the Austins you mention and none of them show the slightest inclination of climbing in my zone 5, with the possible exception of Gertrude Jekyll who is reluctantly reaching out its one and only arm toward a nearby fence. If she would just bloom already I'd be happy (5 years and no blooms so far), and making it to 5' and bushy would make me ecstatic. It might be one of those roses that wants to get to a certain height before it blooms and it doesn't rebound from my cold winters fast enough to reach that height. I planted my Tess at the back of a bed and she still hasn't gotten to waist high after about 3 years. The Pilgrim is a little bigger - reliably 3' or so but not much bigger. Both of them bloom now and then, but they're not in ideal sun. Falstaff is the same size but he's in my virtual zone 6, and hasn't survived at all in a typical zone 5 spot. He had some nice surviving cane this winter and I'm hoping for better blooms this time around. So far he's branching out nicely.

In answer to your bigger question, any time I get a long cane on any rose I tend to try to train it horizontally to encourage more laterals to grow. That increases the blooming if it will put out canes off those long branches - some roses refuse to do this, like my Crown Princess Margaretha - but most seem to try. The down side of having a few long canes stretch straight up is that you might get blooms only at the highest point of the rose. My Poseidon is like that, and I've had to do some pretty creative pruning and encouraging of the canes horizontally to get more blooms down the length of the plant. Chris at NewFlora used to chime in to recommend strategic pruning early in the season for roses like Poseidon to encourage them to branch out more.

Bottom line is that I doubt you'd need to plant these near a substantial support to climb, though in zone 6 you might get more vigorous regrowth after the winter than I do. Pruning and horizontal encouragement helps with more bloom production, but you don't have to do so if you don't want to. It's your choice - while they're young I'd do no more for these than you'd do for any other rose and let the roses tell you if they want to stretch their arms farther than their immediate surroundings.

Cynthia

All those heights and width statements from Austin apply to us over here... it seems to me they simply cut and paste to the U.S. site without any further amendment for different climates/countries.... I can grow all those roses either as shrubs or climbers if I want, and those heights are indications of what I could achieve...according to my climate, my winters and my rootstock...

In my opinion, the descriptions should be treated with caution when growing these roses in other countries.. I would say it's more a case of trial and error elsewhere...

I have Heritage which has upright habit and can be grown as a climber per DA but for me that means the blooms are mostly at the top if I did not train some lower canes more horizontal.

How deep is your border? If the bush is more lax, I recommend you try to pillar the rose around an obelisk to get more laterals and stay within a narrow border. That is what I did with Wollerton Old Hall to keep it within my 2 ft border. See my post on 'Mme Anisette pillared on obelisk.'