can i toggle CSS output to be on for one form, but off for another?

The Output CSS setting is sitewide. I'm not sure if there is a way to disable it on a per-form basis.

Ideally though, the styles for your custom made form would be specific enough that they will always be applied to the elements and never be ridden.

There are some style rules here:
http://96.8.125.10/~trashtax/wp-content/themes/default/style.css

However, they are fairly non-specific and would apply to all fields with these IDs (which can happen on multiple forms) and also the rules in the Gravity Forms forms.css are more specific already so they will likely be applied and these styles would be overridden. If the idea is to preserve the custom formatting you contractor created, you should make his selectors more specific. I will post one example of what needs to be done. You will have to repeat this process for all the specific styles he wants to apply to this form:

Old

[css]
#field_1_19 .gfield_description,
#field_1_22 .gfield_description,
#field_1_23 .gfield_description,
#field_1_53 .gfield_description,
#field_1_54 .gfield_description,
#field_1_55 .gfield_description {margin-left:600px;margin-top:-27px;color:#F3F300 !important;float:left;}

New

[css]
body #content .gform_wrapper #gform_wrapper_1 #field_1_19 .gfield_description,
body #content .gform_wrapper #gform_wrapper_1 #field_1_22 .gfield_description,
body #content .gform_wrapper #gform_wrapper_1 #field_1_23 .gfield_description,
body #content .gform_wrapper #gform_wrapper_1 #field_1_53 .gfield_description,
body #content .gform_wrapper #gform_wrapper_1 #field_1_54 .gfield_description,
body #content .gform_wrapper #gform_wrapper_1 #field_1_55 .gfield_description {margin-left:600px;margin-top:-27px;color:#F3F300 !important;float:left;}

I went a little crazy with the selectors. You just need to ensure that your selector is more specific than the generic default selectors in forms.css.

If you have any questions, please let us know.

If your contractor targeted fields without specifying a form, I would go back to the styles he created, and preface each style with a selector for that one specific form. For example he used this:

[css]
#field_1_19 .gfield_description,

Change it to this to target JUST form 1 (or the form he created):

[css]
body #content .gform_wrapper #gform_wrapper_1 #field_1_19 .gfield_description,

That will make the rules he created apply to just the form he created, and will allow you to use the Gravity Forms CSS for all your other forms.