Milling an additional slot in an arbor press table plate?
My experience with cast iron in the process of milling and lathe turning (minimal, but valid reference) is that cast iron will cut nicely with ordinary HSS tools. The swarf will be granular, not long streams of ribbon and the surface will be moderately smooth, depending on your feed and speed.

A few references on the 'net suggest that it is "self-cooling" due to graphite in the iron. Another few references also suggest that use of coolant will create abrasive mud which will dull the tools.

You won't be destroying your tools.