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.
Correct on both counts.
The staking will become more difficult as the wallets connected online become bigger with coins (whether you are mining, buying or staking), as the more coins you have in each wallet, the heavier you network 'weight' is for staking. Always remember also that a LOT of people out there are accumulating a large number of coins so they can stake more than others, and a lot know how to cycle their coins in the wallet so that a single block can contain ALL the coins in the wallet. This allows a heavier weight for staking as there are MUCH more coins to be staked in one go.
#crysx