I'm building a 7 string guitar. The problem is that I want my bridge to have fine tuning ( like a tremolo has)... does this thing even exists?
with thanks.. MR. Dubin
What he said ^. I don't think something like a fixed bridge with fine tuning exists, simply because there is no use for it.
I would try finding some tuners with a higher gearing ratio if you want finer tuning capabilities. Or use heavier strings.

"Show me war; show me pestilence; show me the blood-red hands of retribution..."
Kahler 2317 is a 7-string bridge with fine-tuning and can be locked into position so it effectively becomes a fixed bridge
Ibanez has the edge fx line of trems but i dont recall ever seeng a 7string model. weird seeing as they have a 6 and 8 string model........