Validation rules can be very handy to ensure consistency in data entry by your users. Validation rules can pose a risk when it applies to all. With integrations like the fundraisingManager forms, the rules should exempt the Guest User profile.
Donation forms may not always create data to match your business rules, but you don't want the integration to fail which results in data loss.
We suggest you add a clause in the formula where the validation rules applies to profiles except the ones specified.
For example this rule will not allow the creation of an opportunity with a probability of more than 50% - Unless the profile is the Sys Admin or Fundraising Profile.
AND (ISNEW(), Probability >= 0.50, NOT( $Profile.Name = "System Administrator"), NOT( $Profile.Name = "Fundraising Profile") )