Mullers & Fred,
I took grade 7 piano this summer and I too had problems managing the staccato scales at first - my fingers just wouldn't do them! However, I perservered and after a while they just started to come together. So, the key is just patience. Stick with it.
C major in 3rds and 6ths are both really easy once you actually start them. I'm sure you won't have lasting problems with these.
There are alot of scales in grade 7 so you may want to spread them over a week therefore completing all the scales every 7 days. I am doing this at the moment with grade 8 and I am just about managing it.
Good luck. I'm sure you will find that once you have been practicing them for a few weeks, the scales won't seem so bad. One tip that I can pass on that you might like to try is rather than practice the scales from the actual ABRSM book which seems very daunting since there are so many to work through, instead summerise all the scales on one sheet of paper so when you come to practice them, all you have to look at is the one sheet of paper rather than the whole book. Of course, you will have to learn the scales first before you can put down the ABRSM book.
Shelton