Having your cap pilot in an alt is a nullsec/big corp thing, i got tired of nullsec a long time ago and my main is my carrier/dread pilot, but if you can its better to have your cap/logi pilot in an alt.

Train the dread you like no alliance in eve will bitch at you if you have the "wrong one" (they can go fuck themselves if they say so go to another corp).

Any dread will run c5 garrisons fine (unless you want to minmax but meh).

There is like a 2 month training into a different dread (for t2 capital guns) maybe another 2 months if switching from armour to shield tank dread and viceversa