need to clarify something here,

since there are no upgrade icons on modifications is it safe for me to presume that there is no limits to the number of modifications permitted on any ships.

and if the above is true its legal to pimp a ship with let's say 10x shield, 10x stealth and 10x anti pursuit

Not exactly. The rules card that came with the large ships says that each ship can have ONE modification in total. (And one title card, but that's different.)

but what about small ships then, or those that means modifications are only avalible to "large" ships

No, any ship can use modifications, but only one each.

Stealth devices on interceptors are particularly effective.