Can anyone make a consumer device that is a phone plug in for salival health diagnosis (inflammation, allergy etc.) This could be a phone attachment where you place some saliva inside the device and it gives you an analysis of various health markers on a smartphone app. Is this possible??

Not trivially. You still need a diagnostic unit that analyzes saliva and creates a readout that has to be transmitted. Another issue is that salivary markers tend to be less reliable than blood.

But the area of lab-on-a-chip research is trying to create miniaturized devices to be able to perform certain bioanalytical operations. Some groups made them mostly portable so that they can be used on the field. I would think that considering the overall size of detectors and other required components that at least a tablet-sized device (if not a laptop) are suitable as they are going to be larger than most current cell phones anyway.