1) No, these interactions target all dwarves. I don't know how I could make it work like that - haven't come across any tags that would let me do that. If someone does know, I'll gladly implement it. I'm currently getting that the probability of you not getting any epidemics at all with your starting 7 is about 70%. Treating them with so few dwarves is easy, just separate them and it'll die out very soon. I'd like some player feedback on if this is too much or not, I'm probably biased myself since I like having my cool interactions flash on the map. Will adjust it as needed.
You could make a "wait" interaction that makes it take one year before an epidemic starter starts the epidemic, but then quarantining migrants, a real-life procedure to avoid epidemics, would be impossible.
2) Currently my suggestion is as drinks, for simplicity's sake. It really is a bit clumsy, though. Store them like you would vauables, and only let dwarves at them when there's need. An airlock system should work for regulating traffic and preventing thirsty dwarves from drinking your potions... that's about it for epidemics. Now delivering blood pressure medication to needy dwarves without permanently hospitalizing them, that's a real problem I haven't yet figured out.
if it were possible to target materials with interactions, then making drugs that dwarves take only when needed would be trivial. I think IT_MATERIAL doesn't work like that, unfortunately. As it is, I think it's either the above or e.g. reactions that generate gas. A third possibility is to make a "doctor" dwarf caste or syndrome that can give the medicine syndromes to others via interactions. You would become one by birth or by drinking a doctor potion (or by a custom "become a doctor" reaction that creates gas that transforms dwarves into doctors).
Or am I missing something? Currently I'm slightly distracted by my attempt to perfect something I started earlier, dwarves brewing creatures into drinks... brewing warmblooded creatures into blood wine and invertebrates into fermented ichor is easy peasy, but apparently how vermin remains are handled means I have to make a separate reaction and material for EVERY SINGLE vermin I want dwarves to make into mezcal (and I want that fluffy wambler mezcal), unless I want one of several bugs
EDIT: If it's something that's good, then I could also easily make this give "profession" syndromes too, such as 1% of dwarves randomly being "doctors" (and while we're at it, 1% could be counselors for psychiatric patients). But I think it sounds a little... stupid.