I suggest that you use a multimeter to measure the AC voltage from the transformer. If it’s below 30V AC it should be fine.
The minimum current requirement is to make sure the transformer can power the circuit and activate at least 1 to 2 valves at the same time. There is no maximum current limit because the actual current depends on how much the circuit draws from the transformer. A transformer rated at 24V, 40VA means it’s capable of delivering 40 / 24 = 1.67A current, but it’s not like it’s ‘pumping’ in 1.67A into the circuit — the actual current it supplies depends on how much the circuit needs. 1.67A just means that in the case the circuit needs to draw 1.67A it’s able to deliver that much while still maintaining 24V voltage. Beyond that the voltage can drop significantly below 24V. Hope this makes sense.
Because AC transformers are un-regulated, in practice higher capacity transformers do tend to have higher no-load voltage. For example, the no-load voltage of a 24V AC transformer may be 28V AC or sometimes low-quality transformer can have a no-load voltage exceeding 30V AC. So when in doubt, it’s always a good idea to use a multimeter to measure it. In any case, when there is a load (i.e some current draw), the voltage should drop down to about 24V.