What are the considerations for veterans serving with a disability rating? How do you navigate staying deployable, ACFT ready etc?

Fitness for entry into service, Fitness for duty, Fitness for continued service/retention, and Fitness for separation are all Title 10 requirements and should not be confused with Title 38 VA rated disabilities. You can be rated by the VA and still be fit for duty and vice versa. They are two separate standards.

From my limited understanding, there is not a magic percentage that limits your ability to continue to serve. I personally know a couple of Service Members that are rated at 100% disability but still wear the uniform. Where the issue rises, is when the disability effects your ability to perform. For example, if I have a disability rating for hearing loss, depending on severity, my PUHLES may read as 112111 which would allow me to continue to serve in most military positions. However, if I have a an award for a lower leg disability, depending on severity, my PUHLES may be 111211, which would remove my ability to serve in many positions in the Army due to mobility issues.

If your disability is severe enough to gain you a 3 in your PUHLES, then a MED board is recommended and they will determine your ability to continue to serve.

So, if you want to serve with a disability rating, continue to train as you fight within the your capabilities and without breaking your profile

https://www.military.com/benefits/veteran-benefits/disability/having-va-disability-rating-doesnt-prevent-you-serving-military.html

Can You Serve in the Military with a Disability Rating?
The answer is maybe. Simply having a VA disability rating does not prevent someone from joining the military. However, the underlying medical condition may prevent someone from medically qualifying to serve again.

For example, you can receive a VA disability rating for knee surgery that you had while on active duty. If your knee has otherwise healed and you can perform your military duties, remain deployable and pass your PT test, then you may be eligible for continued military service.

However, other underlying medical conditions may prevent you from joining the military again. For example, it may be difficult to join again if your VA disability rating stems from a serious medical condition that prevents you from being able to perform your military duties, maintain deployability status or pass your PT test.

If you had a break in service before trying to go back into the military, you may need to process through MEPS again. If you have a VA disability rating or certain other medical conditions, you may need to apply for a medical waiver to join the military.

I also read that you can't get VA disability and Reserve pay so it's one or the other.

If you can meet all requirements to be deployable, then you can be in the Reserves or Guard with VA disability. For the ACFT for those on profile to remain deployable you have to be able to at a minimum pass the deadlift, sprint drag carry and an aerobic event. Unless that's changed...that's what was pushed out about profiles and remaining deployable. If you can't do at least those three, then no you can't serve at all.