No, not unless the Synchro Monster actually says it is, like Formula Synchron, which is both a Synchro and a Tuner. Normally though they are just Synchros.
There is nothing inherent about a Synchro Monster that stops it being used as the Synchro Component for another Synchro Summon. So you could freely use Ally of Justice Catastor and a Lv 1 Tuner for...
The boses are Great Jaggi, Qurupeco, Royal Ludroth, Barroth, Rathian, Gobul, Great Baggi, Gigginox, Barioth, Uragaan, Rathalos, Diablos, Lagiacrus, Agnaktor, Deviljho, Ceadeus [offline only], Jhen...
nefera de nile operetta abbey bominable sprecta von hauntington or most known as sprecta vondergeist deuce gorgon clawd wolf jackson jekyell lagoona blue clawdeen wolf Ghoulia Yelps howleen wolf...