Yes, if you have aero bars that allow the elbow pads to be slid back behind the base bar you can do some or all of your cockpit length adjustment with the pads. if you push the pads way back you might not like the handling when you're down in the aero bars steering with your elbows but moving the pads back a cm or two shouldn't dramatically alter the steering response.
In terms of TT fit and where the pads should be, there are different approaches and one of the tricks is figuring out what approach leads to your best combination of sustainable power, aerodynamics and to some extent comfort if you ride longer time trials. From a big picture perspective the two approaches are:
- Conventional short and low TT positioning where your elbow pads are far enough back that your upper arms are pretty much at a right angle to your back so that you're resting structurally on the elbow pads and supporting your upper body weight largely with the bones in your upper arms. This is pretty standard for triathlon fitting as it's pretty comfortable for long races in the aero bars. How low you go with the pads depends on your flexibility, how far forward you've set your seat to help maintain an open hip angle and the limit of comfort and or being able to see up the road in the aero bar position. Lower isn't always more aerodynamic but for a lot of riders with good hip flexibility, good pelvic girdle forward rotation, fairly straight backs a low position down to the point where your shoulders are within a couple of inches of your hip joints (or more precisely the origin of your lats is level with the outside edge of your shoulders) is usually pretty aero. Much lower than that and you start showing less chest to the wind but exposing more of your upper back and risk both a lot of power drop as well as a lot of neck soreness trying to see up the road without additional aero benefits. In this position the pads have to be pretty far back and the overall cockpit is pretty short so given your TT frame is actually longer than your road frame (is the top tube still longer when riding in a steep seat tube angle, saddle forward position) you might need both a shorter stem and pads slid behind the base bar.
- High and long 'Faux Superman' or 'Deep Diver' position where your base bars and aero bars are higher but your pads are further forward and your upper arms are stretched forward and not square to the angle of your back. In this position the seat angle is generally similar to your road bike seat position, the bars are higher but you're stretched out longer but within the legal UCI limits if that matters to you (i.e. you do national level events at least here in the states or you race in a country that adheres to UCI regularions). Your torso and hip angles shouldn't really be much different than the other position but you get there with higher bars and more forward stretch coupled with a slacker effective seat tube angle. This position can work for shorter to medium length time trials but might get real old for five plus hours of racing during something like the Ironman. If you go this way you will be somewhat stretched out and the pads will be further forward. It also has the advantage of leaving your seat angle unchanged or at least closer to your road bike position so there's less adaptation between the two positions which is nice for someone who mostly trains on the road bike. Whether this position is sufficiently comfortable and or is at least as aerodynamic depends on you and your flexibility. Personally I'm far more comfortable in a high and long position, my power drop off relative to my road bike is much less (as in within a percent or two relative to the 5 to 7 percent drop off I see early season in a short and low position before I've adapted to it) and surprisingly both aero field testing and race results tell me I'm more aero in the high and long position but this obviously doesn't apply to everyone.
Here's some screen shots taken from fitting videos I took a while back comparing my best high and low position and even though the high and long position(top photo) looks a lot less aero, it's actually faster for me even at the same power and I tend to get a few more watts there as well.
Anyway, yes you can slide your pads back if you need to. The biggest reason folks don't do that all the time is that many aero bar designs do not allow you to slide the pads back so it's often not an option unless you use certain brands of clip on bars.
-Dave
