Yeah, I wasn’t sure if you can stack Frenzy with your 0 cd skills, but since @xechidna says it’s bad I would rather believe her(or him?). Since she is really experienced fencer player.
Yet I don’t think peltasta is THAT mandatory, C1 won’t make you as effective as C3 tanks. So you can stay with Highlander and perform as secondary DPS dealer in a party with another tank. Highlander can slightly increase your DPS through Cross Guard Vulnerability buff: it doubles pierce damage during next few seconds.
I personally won’t advice you to choose one over another, both are good but they fit slightly different roles.

If any1 have any questions about it, just lemme know!