Looking at the English text, it really is a very polite way of asking someone to only test a certain type of device of product on the system. If you use the literal translation, "si fuera posible" you're not conveying the same idea in English. The use of the imperfect subjunctive (fuera) indicates that it is impossible, which is not the idea that the speaker wants to convey in English. The idea in English is that it IS possible and they want to remind the user that they have to be careful when testing the device.
A better translation of this phrase would be using the structure "Le rogamos." "Le rogamos que sólo pruebe herramientas informáticos....en un sistema aislado o en una red aislado."