Tripod problems...

So i have my dads old Slik tripod that he used for his Nikon fg, and i used for my pentax Ist ds. But my problem is when i try to put my 30d on the pod the bolt is much to small to fit the camera. I thought the tripod mounts where the same size on every camera. Now i'm looking at picking a new one up is there mount size differences between each tripod? Also any suggestions under 100$?

If the tripod fit the Nikon FG, then it'll fit the 30D.
Screw is 1/4" in both cases.

Screws for attaching camera's to tripods / quick release plate come in 2 sizes, and two sizes only: 1/4" and 3/8". The bigger one for some medium format and large format cameras, the smaller one for anything smaller.