I know alzheimer's can be caused by excess dopamine, and ADHD by not enough dopamine... So if I take Adderall (dextroamphetamine and racemic amphetamine) for adhd, am I at risk of alzheimer's?

Probably not. The current theories of ad emphasize reduced synthesis of the neurotransmitter acetylcholine along with amyloid plaques & neurofibrillary tangles inside nerve cells that disrupt neural functioning. Ad individuals also lose locus coeruleus cells that generate norepinephrine. Other theories suggest possible inflammatory and glutamatergic roles. Parkinson's is assoc'd w/ Dopamine loss, not ad.
AD neurotransmitters. Ad is more complicated that a simple neurotransmitter imbalance. Communication between brain cells is compromised in ad, dopamine, acetylcholine and glutamate all have a role to play. While some cortical pathways are shared in add and ad, one does not necessarily increase the risk of the other. Maximally treating your add including non-drug therapy is critical to maintaining a healthy brain.
No correlation. Alzheimer's is due to a combination of many different factors but excess Dopamine is not one of them. There is no correlation or known risk between taking Adderall (dextroamphetamine and racemic amphetamine) and developing alzheimer's.