I think 1 inf/med tank and one heavy tank should be the limit. Tanks like jagdtiger, tiger ace, elefant and isu, should take up both tank allowances.
Yeah I am sure your right. And to add more speculation there will probably be some sort of drawback to using a customer commander over a standard one. For example if we assume T34/85 Pair is Legendary and Mark Vehicle is Legendary then you can have one or the other not both. Unlike Guards motor for example.
There have been games with custom load outs before and this is more or less how they were balanced. It makes it impossible to cherry pick the best and makes the boiler plate builds still viable.