The castle flying on the top of a storm cloud sounds quite cool.
But about the effect to manage that and the Elementalist virtue, I don't think that the virtue is crippled either; it's just that as written gives more advantage if you take it the other way and use it not with an elementalist, who is supossed to have high scores in every elemental form, but with a specialist in one elemental form that can then cast Muto spells to other elemental forms without bothering on their scores.
I thought a bit about here, while doing a Verditius focused on swords and taking the hardcore approach to a focus in Swords (that it applies when you cast spells on swords, non in anything where a sword is involved). This verditius is on paper the opposite of an elementalist: he is a MuTe specialist, who turns Terram stuff into other elemental form.
As for your spell, I think that in order to support stuff on the top of it, your cloud doesn't require the Terram requisite. On the Terram description it's written that it affects solid objects, which is not the same that object's solidity. To make the cloud solid, I would require only CrMuAu (if the same effect creates the cloud and makes it solid) or a CrAu and MuAu (to create the cloud and then to make it solid). Looking at the Muto forms in the sourcebook I just see Terram requisites where you actually turn things into stone (like Rain of Stones, which turn raindrops into stones, or Trapping the Fire, where you turn fire into a gem or stone). But Talon of the Winds, for example, turns wind into abrasive stuff which must be somehow solid to make damage without any other requisite than Rego to point it around. With this point of view a MuAuTe spell would actually turn the cloud into stone, which would give you some issues with flying maneuverability in anything else that flying down at full speed until hitting the ground (hmmm, that would be a nasty offensive weapon for such a castle, anyway: turn other clouds around into massive solid cloud-bombs...).
And that would make the Elementalist virtue quite useless to your character, wich on the good side would mean 3 virtue points to spend in something like a Major Magical Focus in Weather or even better Affinity with Auram, Puissant Auram and a Minor Magical Focus in either Stormclouds or, if you rule it narrow enough for a minor magical focus, Storms.