Do you think 7500zl brutto would be enough to rent together and live... and (maybe) to put some save on it ?
That's 5000 PLN net. About what you should be making on your own to make the move worthwhile. Rent with all fees and utilities will set you back about 2000 to 2500 PLN a month. Food will cost about 1000 PLN if you eat solely frugal homemade meals cooked at home. Double that or even more if you like to eat out or eat a lot of expensive things like prepared meals, meat (beef will be a rare treat), or foreign foods. You'll also have to take into account 160 PLN a month for public transport (you couldn't afford a car), plus the fees for you airfare, residence permit and health insurance.
If you are a recent grad in search of a year or two of on-the-job experience, it might make sense. Frankly, though, there are certainly other opportunities out there that are much more attractive, including using the time to beef up your salable skills and qualifications in your home country. If you are not a recent grad, then it won't make sense at all.
A word of warning: the "foreign-boy-moves-to-Poland-to-be-with-Polish-girlfriend" story rarely has a happy ending. Plenty of threads about that on this forum. You're more likely to succeed in your relationship outside of Poland, for a whole stack of reasons.