I'm not quite sure whether by 5a, 10a etc you mean the location of the fuses (are there locations 5a ?) or the fuse rating in amps.
If you mean amps then using a 5A fuse instead of a 6A fuse should be fine. But putting in an 18A fuse instead of a 10A fuse isn't a good idea - if there is ever a fault condition, an 18A fuse could allow 4x as much power as a 10A fuse and might cause trouble. In the UK getting 10A fuses is a doddle - maybe you can find another dealer?