600 simple text expansions would work fine in Keyboard Maestro. The internal engine for doing the matching is very efficient. Managing them would be ok, but not great. And they would take space in your macros file, but probably not enough to be a problem (each simple text expansion macro takes about 200 bytes currently).
So yes, if you don’t use any of Text Expander’s features, and just use simple expansions that Keyboard Maestro can cope with, then it should be fine.
But I’d still generally recommend you simply use both programs and let them each do what they do best together - 600 expansions is quite a lot, and its likely that if you use that many you’re going to want to use more, and want to use some of Text Expander’s many text expansion-specific features.