You can dogfight in the pony, but with a couple rules of thumb. If it is something like a jug or a 190 then it will be a pretty close turning matchup, and you can fight them relatively normally. That is to say you can angles fight them and especially use your flaps to decelerate quicker and get a tighter turn before they can. A K4 is a tougher opponent, and a major threat as it has the power to beat you in an angles fight if things get slow. You have to be patient when flying a 51 and break off if things get too slow with a K4, because a Pony with full flaps out for too long will simply wallow. Your turn rate will take a major hit, and you'll have to just put the nose down, get some separation and reset. Restart the fight fast and use those flaps to get an early angles advantage and make it count before things get too slow again. You can also use this method against many other planes, for example I've done this tactic to a Niki, but it took longer because it starts out with such a large turning advantage.
To fight some of the better turning planes you simply have to E-fight them, as if you get slow at all they will come around on you and most likely out-accelerate you too. Use your top speed advantage to come into the fight fast and then use that to get above them and get an E advantage. Then maintain that advantage through high yo-yo's etc. as you slowly work them lower and lower until they have no where else to go and no E to work with. Be careful with certain planes, especially the Ki-84 and Spit16 as they have the uncanny ability to equalize E states very quickly. You have to stay aggressive, stay close, and stay behind their 3/9 line so they aren't given a chance to gain energy.
On the defensive, I utilize my high speed flaps a lot to force overshoots on planes that don't handle well at high speeds. The 51 can put on the breaks pretty well so you can force some overshoots, but be careful as the 51 also accelerates very poorly and can be killed if your opponent doesn't fall for the overshoot if you don't have some room to dive away. Otherwise use your speed to get away and reset the fight on your terms.
However, the most important aspect of succeeding in the pony is situational awareness. I like to come into fights at about 8-12k to start out with some altitude to work with. If our base is capped I will come in at 16k as the pony doesn't handle as well between 12k-16k for some reason. Anyway, always be looking around as higher cons can quickly spell death if you lose track of them and they end up on your six, with little options as far as outturning goes. You will be forced to try an overshoot which will leave you vulnerable for other cons. If at all possible drag single cons out away from the main flow of traffic and fight them 1v1 where they don't have help waiting to swoop in and kill you.
