Some of deca will convert to estrogen. It is a low conversion rate however.
No one has conclusively found that Dostinex will prevent progesterone gyno, which makes sense considering dostinex is for treating hyperprolactinemia or prolactin.
Once again, the only thing I have seen that makes claims to reducing progesterone is formestane (which will also stop aromatization).
What dostinex users do report is an increase in sex drive, which is good considering 'many' deca users also report 'deca dick'. It is also reported that recovery is made easier by running dostinex through the deca cycle.
As I recall the mechanisms of which prolactin and progesterone cause gyno are somewhat more complicated than estrogen's effect on gyno.
You will be fine without the dostinex, people have been using deca for years without it. Its only up until late has paranoia about its effects have run rampant. Use HCG though, progestins will make recovery harder.