You have the ability to block immediately at the pull with passage of arms. So long as your aoe mitigation is sorted out you should be able to plan for a passage later on while Veil+ co-tank mitigation covers anything in between. Whoever pulls should be determined by strat and tank invul placements.
As for gauge in the beginning, the topic is about generating gauge during downtime, not starting off with 50 gauge just for a block, cover, intervention etc.
Leviathan is pretty interesting because you can Hallowed ground the first tank buster which obviously negates any issues with gauge. However, Leviathan does have some small downtime with Maelstrom but your CD's are so plentiful that you can just pop rampart and sheltron for Tidal roar's + auto attacks.
Titan is a fight where you overflow on gauge anyways unless you'd rather spam sheltron for autos.
Burning cooldowns and then not having it when you need it is a problem for any tank. All tanks need to properly plan out cooldowns. This doesn't just apply to PLD you know. What makes it different for PLD is that during downtime phases your gauge halts while other tanks have their short CD's on the ready. You can of course have sheltron where you need it but you may not have generated the gauge you'd like to have intervention, cover, chained sheltron etc. Also, if you're forced to spend gauge before downtime and then require gauge after downtime this of course goes against what you just stated.
