Do all things that are tamei have to be more than an adasha or just a sheretz? for example is less than an adasha of zera tamei?
There is no general rule. Some things need a minimum of a lentil's worth, some a barley's worth, some an olive's worth, etc. You just need to check each case. For example, check out Mishna Ahilot Chapter 2 for various minimum sizes related to corpse-impurity.
Regarding your specific example, yes, the minimum size of semen to impart impurity is a lentil's worth (Rambam Shear 5:2).