You can really use any type of acid if you are salting into an aqueous mix and then basing. It won't effect the process much. DMT fumarate is one of the more popular stable salt forms that form a crystaline structure at room temp - which is why many teks use it. It is also safe, and over doing it won't be as quick to cause severe chemical burns (which is easy to get if you mess with a concentrated strong acid with out appropriate equipment and safety procedure.)
When you use an acid, really all you are looking to get from the acid is it's available H+ ions. Which, when dealing with an acid like phosphoric, will be triprotic and have three H+ ions that disassociate from the phosphate group at three different pKa values. It has three different conjugate bases which will appear at each dissociation. If you were to fully dissociate phosphoric acid, you would end up with DMT-phosphate (eventually) but you are likely to only hit one or two of the pKa needed to dissociate when doing a salting.
The acid-water ratio will depend on the pH that you wanna use to salt. Phosphoric is a strong acid, and won't need too much to lower the pH enough to salt the DMT. You don't want to saturate the water with this acid. It is not like fumaric which is a weak acid. Add just enough to get the volume you are working with to the pH you desire.
There is a way to figure out what amount you will need per volume of water, but you would need to share more about your desired project to be able to assist with that - I.E. how much water you want to use to salt, what pH you want to reach, etc.
Sometimes it's good for a change. Other times it isn't.