your questions regarding the game rules (improving the interpretation).

Yes, upgrading fairies is not a good idea. But with psionic creatures, even if they gain magic resistance, they lose the ability to use their own magic.

I don't mind removing the ban on upgrading psionic creatures, but the cost of such a unit is 800.
The unupgraded version costs 750, deals 10-20 damage, and has a speed of 7.
The upgraded version costs 800, deals 15-25 damage, has a speed of 9, and is magic immune.
If the unit's cost is increased to 1200, upgrading can be allowed.