Cross-skills and main skill

Ok i love the whole cross-skill system but there are some flaws that i don't really like about it and more when you do a lower level dungeon.

For example i have lvl 50 whm and lvl 26 thm.
If i go to Sastasha for example on my whm i only have the skills that my whm got till lvl 18 since it is capped on that lvl. So i have no stoneskin since i am capped.

But if i go on thm and i use cross-skill i could equip stoneskin and use it in the party.
So i go in the dungeon as whm lvl 50 get capped can't use stoneskin and blm or any other magic class goes in who leveled whm to 34 could equip it as cross-skill.

I find this not the best thing why can't it made so the skills tune down in either power and mana usage or not allow other classes to use it as a crossclass skill.