lots of questions there.. and 'll not try and answer all of them because i've forgotten most of then now anyway... oops...
with brackish you sort of are stuck between GSP and Fig 8.. then BBG's, knight gobies etc..
and with your tank being 0.004 SG? (how can that be possible... i guess you don't drink your tap water right? ICK!) you may struggle to keep fresh water fish... (or do you keep them just fine?)
I think you need to be a little braver and start a small marine tank... it really isn't as scary as you think, and you could have a specimen toby tank... like the most beautiful, packed full of personality valentini... he'll do great in a 30G and you may be able to find a smal goby type to go in there as well.. once the tank has matured out a bit..
and if you keep it fish only with live rock then i doubt you would have any problems, you are already competant with your parameters, and with fish only tanks it takes most of the stress of marine keeping away, it's the corals that tend to be fussy, valentini's are no more fussy than brackish puffers.. just like it a little saltier.. top up with fresh when the tank over evaporates and your fish will stay happy...
and i think you said you had GSP's? well they will appreciate full marine at some point, so now might be a good place to start learning...
I started a 30G marine tank after i got a GSP, knowing i'd have to take him to full marine, started slow and small, and after getting more confidence i expanded, now i have a 60G marine tank (with a few starter corals and my juv. puffers) and am upgrading to a 100G tank soon...
eventually i want to combine my marine tank and my brackish tank into one huge marine..
if you go this road you'll have to be a little more patient, and up your salt by 0.002 a week until it reaches 0.023/ 0.025...(or re-cycle it marine) but it would be so worth it when you got that valentini/ other toby in there....
you don't get over the beauty of a marine tank fast...
