There is no way to gain troops before owning a colony, but you can gain colonization ships if your explorer ships manage to find one buried on some planet. Pirates acquiring empire capitals through criminal networks is also working as intended.
As for pirates building construction ships, pirates can do almost everything a regular empire does, it just starts with no planets and has to go through the Pirate Base>Pirate Fortress>Criminal Network building chain in order to acquire a planet, which is extremely expensive (~$350,000)... Note that criminal networks are considered the pirate 'end game' so to speak.
The main way that pirates are limited is diplomacy, Empire AIs are far more aggressive against pirates and pirates do not have the myriad of diplomatic options of peacefully dealing with its neighbors. Infact, most of my pirate games usually devolve into a war of attrition with empires constantly sending suicide fleets against my defense.
Other pirate limitations include being unable to build research stations, being unable to investigate or benefit from ruins, unable to benefit from research wonders, unable to get Ambassadors or Colonial Governors, and they gain little benefit from their initial race pick. Given a few of these restrictions are probably bugs, but they are still there.
I ate the batter of the bulge at Hans' Haus of Luftwaffles