I mostly pgreelay 1vs1 and I don't think that a Puma comes too early because it is the only thing that helps vs ami AA HT, Greyhound and M3 with flamers (when it is good managed it can easily outmaneuver raketenwerfer). With a RAK, you can maybe scare them away but with a Puma you can actually kill them.
1vs1 is usually about CALLins. In your fuel calculation, you should not forget that Allies have solid CALLins, while OKW has nothing (Elite Armor doesn't come out of the box and you need to spend 210 fuel on them). The Puma is the only thing that keeps you alive in 1vs1 until you save fuel for heavier armor.
As Soviet, who techs to t3 when he/she can go T34 CALLin. As US Force, you usually go to Easy 8 which is an absolute allrounder.
Absolute agree about the Callins. They are just about a little bit too attractive because you spare so much tech cost. One more reason for adjusting tech and CP costs.
Don't agree about Puma. The Luchs should be the first OKW tank and it should counter the US AA HT (lower its pentration), Greyhound and so on. A M3 can really be countered without a Puma, this is overkill. The Luchs should be compareable to T70 and Stuart and all oft hem should arrive before the Puma does. Exchange Luchs and Puma in tech structure, adjust fuel costs so that Luchs, Stuart and T70 arrive at the same time, lower penetration of AA HTs and there you have a time window for small tanks that counter light vehicles of course.