The eureka zanzibaris are a couple mils larger than than foundry range, in fact they fit in almost perfectly with the copplestone range heightwise
The rifle armed ones would be too late to fit in with a pirate force but the rest would be fine. I'd recommend them.
Ah, sorry, just reread the question:
Yep, the foundry zanzibaris would fit in well with the eureka pirates.
Most of the foundry zanzibaris are wearing robes which would make them stand out, but other than that they would be good. (they are a good size match for eureka and stick to muskets)
Other darkest africa figs would go into a pirate setting well, the ruga-ruga would go with arab style pirates and add a bit of character