Is possible do solo mining with the gpu?
Yes
And another question I have 7 days for the next stake and one friend of mine with less coins has 14 days, if we put it together we take + stake or the same?
If you increase the amount of SWEEPs on the staking wallet the staking time will reduce. Thus after putting your SWEEPs and your friend SWEEPs in the same wallet for staking you will receive stake reward more often.