There are many types of relays made that are the same size ( body), however they are all rated at different operating amperages. What manufacturers do is change the material of the relay contacts and pins that will allow them to run at higher temps and amperage. For example they can use copper, silver and in some cases gold plated depending on the specs the relay is built for. There should be no reason why you cannot use a 30 amp relay for common fuel pumps on the market today. But I would reccomend to size your relay amperage rating accordingly, to the amp draw on the fuel pump rated by the fuel pump manufacturer.
Another important note is the wire size that is supplying the relay and going to the pump that is just as important, a 10 gauge wire is sufficient for most pumps we work with in race cars, and there must also be a 10 gauge equally supplying a ground to the fuel pump preferably from the battery or a common grounding post.
Usually you would need around 8 volts to activate a relay, and it would have to see less than 4 volts to deactivate them.