Does SSI count as support when trying to determine dependency of an adult disabled child? It is enough for half expenses, but does it count since it's SSI?

The Social Security benefits received could be used for their own support. 

The question would be did the proposed dependent actually use the benefits for their own support?  If they did and it was more than half of the support then they cannot be claimed as a dependent.

Yes, according to this Turbo Tax source, it can be used as a dependency test to determine if this dependent can be claimed by you. If it contributes to more than 1/2 of their support, then they cannot be claimed as a dependent as DoninGA mentions.

@wendyheck