Yaks are one of the most versatile planes in the entire set. They turn pretty well above 200mph (or so I hear- my experience in them is very limited). As a stereotypical newbie who was interested in German planes, the Yak and LA7 are both tough customers for any of the German planes. The Russian WEP doesnt work above 8,000 feet, but that only really matters if you really need the extra speed.
The Yak cruises at about 380mph at 15k- in other words, it is damn fast at that altitude. On the deck, it isnt as fast- it is maybe the 4th or 5th fastest plane on the deck. I know a runstang will outrun it (p51d), I think a Dora will outrun it, I know a Tiffie will, and I know a 109G10 will on WEP (well.. I think anyways, I've seen it go both ways). However, the Yak can outturn any of the planes that are faster than it is on the deck (oh yea, the LA7 is faster on the deck as well). The firepower isn't so great on the Yak, and the fuel doesn't last very long. That said- it IS a very solid fighter at low to mid altitude.
As far as tactics go, you will have to match your tactics to the plane you are fighting. I can try to give you some general guidelines, but my experience in the plane is pretty limited. I'll also try to keep it to the most popular planes to shorten it up some.
Niki- I have absolutely no idea on this, don't recall fighting a niki in a Yak. That said, the Niki turns extraordinarily well (only the Zero and Spit turn better), so you'd be well advised not to turn fight with it. I wouldn't try to out zoom one either, they are very hard to outzoom. I'd say keep it on the deck, try to stay fast, and hope that help comes.
Spit- The spit will outturn you, but you can hang with it for a little while (personal experience only, not sure if this is always the case). Again, try to stay fast, use the vertical as much as you can, and try not to get hit by the hispanos.
LA7- You *should* be able to out-turn the LA7 at low altitude, and I think the Yak rolls better as well. If you can get the LA7 into a turn fight you should be OK- if not, use a scissors or rolling scissors to try to force an overshoot. As an alternative, try to get the fight low and slow, I hear the LA7 handles like a pig at 150mph and down.
109g10 and 190d9- I'll assume that the G10 is faster on the deck (although I have seen it go both ways). You can outturn a 109g10 at all speeds until around 150mph (personal exp.) The roll rate on the Yak is pretty similar, maybe a little faster than the G10. I'd recommend getting the fight to between 200-300 mph and trying to get the 109 to turn. If a 109 gets on your tail, head for the deck, and once you get to 400+ mph you can do your evasive manuever of choice - the 109 can't follow you due to compression. The 190d9 is faster than the 109, but it doesnt turn as well nor does it accelerate as well (the Yak outaccelerates both of them). I think the Yaks climb rate is superior to the Dora, so use the vertical while fighting them- you'll catch up in E eventually even if they bounce you.
P51D- The yak can outturn the 51d no problem. Most P51 pilots will make one pass and head for the hills, so you never really do get to "dogfight" with them per se. The P51 is a LOT faster than the Yak on the deck, so you'll never be able to catch up to em anyways. I'd recommend just avoiding their bounce and then going your merry way.
I think I covered just about all the oft-used MA planes. If you are having any particular trouble with a plane, lemme know and we cna head to the TA or DA and mess around with it.