What is proper voltage for surge suppressor for home? Should each power outlet have a surge suppressor?

Is it safe to connect an extension power cable to suppressor?

The basic surge suppressor/power bar is protected by simple MOV devices which generally last about 2-3 years given most homes power supplies and brownouts/spikes from other household items like furnace fans cutting in/out and compressors from refrigerators. After that, they are basically JUST powerbars.

Connecting a extension cord to a suppressor/power bar is no different than connecting two extension cords together. Any extension cord should be rated for the load it is going to carry. If your extension cord is just plain zip cord (18 ga) then it's only good for light loads like a lamp or radio (not near water). My cords are all heavy duty 12 ga. for things like power tools or delivering power over longer distances.

Extension cords are only for temporary usage. If you need more power for something - ALWAYS use a new properly wired permanent receptacle.