Duck-
I personally love the 300 as a test, and include it in all testing I've done, in addition to a 30fly, to give us a scored heptathlon that we've extended out from the LSU pentathlon scores.
I think yes you can test with what you've done. In my opinion, 300 is a good distance for all athletes on the team (minus throwers), because it falls just south of huge lactate accumulations, so short sprint and jumps kids can even perform it decently. 400 types can crank on it also.