This would be pretty fun actually, a fourth race would make the 2 vs. 2 match up even better.
I do hope though that you'll make the pirates as unique as the other 3 races are. (as far as tech tree, cap ship special abilities etc..)
From what I've seen the majority of their ships are TEC with spikes added on, (They remind me a bit of the Reavers from Serenity)but their complete lack of capital ships leaves them woefully underpowered as far as a playable race goes; which is something I hope you'll fix if you do enable them. (seeing a few ships from each race would be kinda neat since they are pirates after all and get their ships from probably everywhere they raid)