I think that they said that startgates on board ships could only be used when the ship is on or near to a planet, so that the 7th chevron could be engaged.
As to the speed of Apophis' cheops war ship, I think it would have to go alot faster than 10c.
SPOILER
In Exodous SG-1 were flung 4,000,000 light years from home by the explosion of a star and they were told by JAcob that it would take about 125 years to get home. This corresponds to a speed of about 32000c, if the ship was capable of travelling at high speeds indifinitley. If not then the top speed might be much more.