A Tekkit server and a TekkitClassic server are different and if you really want to find a server pluging for an earlier build it should bein the corner using the drop-down arrow, but yea you can choose which server plugin for the build you want to use but TekkitClassic is a completely Different modpack. Aka if your getting a Tekkit server plugin and not a TekkitClassic plug-in you may be downloading the wrong one pm if you need even more depth help I do not mind.
-Fallen