The three tank class could work i think, it's less complicated then the fps limit, different colors are easier to regulate then different fps too. And to answer the concern over finding one small tank with a 76+ mm gun i don't really see that as a problem.
At least not for ww2 tanks, as all were either 75mm, 76mm, or 88mm for main battle tanks. But there might be concern over the 4.5 m long stuarts, although they're armed with just a 37mm gun. But the Norinco Type 62 light tank is only 5.5 m long, and armed with a 85mm gun, the Giat amx-13 light tank with 90mm gun and 4.9 m long, and the alvis scorpion reconnaissance vehicle with 76mm gun and 4.8m long.
Those lengths are the hull length, which i should also point out. Most of the tank books, with the only exception i know of, Janes, publishes the lengths as total lenth, that means with gun forward. For example, the m1 abrams hull is 7.9m long, but with gun it's 9.8m long. If one wanted a 90cm tank, about three feet, one would have to watch out for the lengths. If one used the hull only lenth it'd be around 1 : 7.7 scale, but with gun it would be about 1: 10.9 scale. Just as a warning when designing and building.