Is the mercury in vaccines tiny? Is it less than the amount you get eating fish?

Yes. The form of mercury is also one never proven toxic to humans. Its human effect is similar to the difference seen in alcohols. Methyl alcohol is so toxic a tablespoonful will kill you (if ingested), while ethanol can be taken in rater large quantities & just get you drunk.
Yes. With the exception of certain types of flu vaccine all the vaccines we use are mercury free. 1 can of tuna contains more mercury than 30 flu vaccines.