Is aloe okay or helpful to put on a skin rash caused by an allergic reaction?

Yes; try cortizone (hydrocortisone) Aloe is generally very soothing to most rashes. If your rash is due to an allergic reaction, try otc hydrocortizone cream a few times a day as needed to alleviate the rash and any itching. You can also soak in an aveeno oatmeal bath. Oral Benadryl (diphenhydramine) can be helpful if itching. If the rash is getting worse or seems painful, contact your doctor to take a look.
Yes. It may soothe skin and is rarely irritating. If however rash is severe you amy need prescription strength creams.