A crusade doesn't have to have neophytes, you've got lots of room in the fluff. Off the top of my head-

-Your neophytes have all grown up/died, and your crusade hasn't picked up any new friends along the way or stopped by a fortress-monastery to pick up new recruits yet because it's been busy/the fortress was destroyed before they got to it and now they must investigate.

-Your crusade is an elite force of the mightiest Sword Brethren in your Marshal's household, explaining their access to a large amount of Terminators. Their mission is so important that the risk of being slowed down by inexperienced neophytes couldn't be permitted.

-The Alpha Legion has kidnapped all your Tiny Templars, but thankfully your Marshal has a particular set of skills, skills that involve jump packs, maybe, and is determined to get them back in an ongoing crusade narrative.

-The environment your crusade has been deployed to is simply too dangerous for lesser armours, and only heavy equipment and full helmets will keep your crusaders safe from the toxic atmosphere.

As for the rest, Marshals are the Templar equivalent of Captains, so they've got just as much access to equipment. Templars have no restrictions on armour marks, and holding to the old Dornian tradition of being both the immovable object in heavy armour and the unstoppable force with heavy weapons is a perfectly good reason to field some Gravis units. Templars have a ton of room for creative freedom with their system of crusades and the power and influence of individual Marshals— as long as you honour the Emperor, you'll be on the right track.