HITH is an internal protazoan you are correct. It's not generally advisable to mix anything other than Kanamycin with erythromycin and even then it must be done with close monitoring. Parasite gurad contains Praziquantel, Dflubenzuron, Metronidazole and Acriflavin and while this is a good medacation I wouldn't mix it. My suggestion would be to finish treating with the erythromycin (since it is helping with the eye infection), do a 50% water change run carbon for a day and then use the parasite guard or API General Cure, if necessary. Does the sore appear red or is it white like it's healing?
The meth blue which you couldn't get would have been helpful and works well as a bath when treating this with the erythromycin.