There mins are etched in stone. You must have 1000 or 1500TT to be hired as a street captain.
As far as receiving accurate and pertinent information about Gulfstream, I would think that it would behoove you to call them directly and ask the questions that you have posted. There are one or two guys that participate in posting comments on this forum that fly for Gulfstream or at least supports those that do. Most here do not like Gulfstream b/c the company offers you a guaranteed job after completing their "First Officer Training Program." It is referred to as "Pay For Training" or PFT and is looked upon as paying for your job as oppose to competing for the positions based upon TT, experience and and like.
Although it is true that you pay for the training in either a 1900 or 120, one should be able to secure the right seat base upon the number of hours that is required to fly those a/c. Other regionals have mins as low as 300 hours, which seems to be the norm, for a/c such as the 1900, EMB-120, Dash-8, ATR-72 and so on.
I hope those that do fly for them are able to offer you some sound & viable information. Btw, the bases are Tampa, Key West, Miami, Fort Lauderdale and West Palm Beach as per their website.
atp