Well first of all its probably helpful to understand that Dexamphetmaine and Ecstacy (assuming ur lucky manage to find an MDMA pill) target different neurotransmitters, hence the different effect between the two.
Dexies - main effect is on dopamine - these are your "reward" neurotransmitters, its easier to concentrate on mundane tasks (like studying) when u've had a couple of dex because your brain is full of dopamine, it feels rewarded for doing stuff it normally wouldnt like.
Ecstacy - releases serotonin, this is your loved up, feel good neurotranmitter. It is responsible for happiness, well being etc. Lack of serotonin is thought to be a cause of depression.
Both drugs release the neurotransmitters into the synapse, so therefore there is bound to be some recovery time required for the reserves of either dopamine or serotonin to be replenished
Just a bit of background so you know whats going on.
On to your questions
1. Are dexies detrimental to your health
Technically yes, but if your doses are relatively low and you are not taking them very regularly then the damage to your physical health is going to be almost minuscule. You might want to watch out for psychological dependence. If you begin to use them more and more to do assignments/exams, you might come to think you cant do the work without them and will end up wasting more and more time sourcing the drugs and could potneitally lead to harder drugs eg. METH. Take them for long enough and your dopamine system downregulates in your frontal cortex, you could potentially give yourself ADD like behavoiour issues.
I am prescribed 60mgs of dex a day so if thats okay to be prescribed then i assume its not going to kill me. I do take regular breaks tho, because it does place your body under constant stress and this will show up in things like bad skin, coarse hair. etc.
2. Dexies and alcohol
Depends, dexies are metabolised in your liver, so is alcohol. Both at the same time will put your liver under more stress = bigger hang over. But its not going to kill you. You will find that drinking with dexies, you wont feel as drunk, so you need to drink even more to get pissed. I very rarely drink anyway so i cant really help much on subjective experiences.
3. How often
Dexies are nowhere near as harsh on your body as X. As long as your not downing stupid amounts of Dex, you could just swallow em and forget about it. With X you have to be careful, much more chance of overheating, potential neurotoxicity etc. Ecstacy you need time to recover from. I wouldnt recommend doing X more than once every two weeks and even then i would advocate a longer break in there. Ecstacy was never really my DoC so im just going on what my mates would do. Ecstacy definitely more harsh on your body, plus Dexies are phamaceutical quality so you know what you're getting.
Ha. Reading over this sounds like im sales rep for Sigma Phamaceuticals try to get everybody on the D-train. Wasnt sposed to come out that way, but really, you need to take A LOT of 5mg pills to do yourself some serious harm