Just for future reference, benzos aren't recreational on their own, if you are abusing them in this manner, you shouldn't have them IMO

save them for something they're useful for, like anxiety, sleep, seizures, and stimulant comedowns. Abusing them just makes you really tired and stupid for a few hours. And
especially don't
ever combine them with alcohol. Ok, that's my piece on benzo politics. Lol.
Well anyway, I'd try taking .25 mgs of clonazepam with your MPH, because the form you have, concerta, is an ER medication so it lasts a while, and alprazolam is fairly short-acting, which could be problematic because of rebound anxiety, while clonazepam has a very long duration of action, possibly out-lasting the MPH, thus soothing the crash, killing two birds with one stone. If you are still having anxiety, it should be safe to take the other half of the .5. Don't dose too high though, not only is it dangerous, but it can kill the high. IMO, stimulants and benzos, when properly balanced, are perfect, although I don't have much experience with DRI's like MPH (or cocaine) mixed with benzos, but I did mix them with other depressants such as opiates and alcohol, which I do NOT recommend, however judging by my experiences with those combinations, benzos should mix well with MPH. My only warning is to be very careful, as MPH is really hard on your heart, and mixing it with too high a dose of benzos could end badly, although since you said you don't dose high, you should be perfectly fine.
IMO, however, the anxiety is simply because you weren't entirely familiar with the drug's effects, so the sudden rush of exhilaration startled you, and when it leveled out, you were fine. Try taking it by itself one more time before resorting to benzos, I think you'll find that the anxiety will be diminished, but if not (which is likely, seeing as this is MPH, the dirtiest stimulant there is), follow the instructions I posted in the second paragraph of this post. If you don't have any considerably intense anxiety, save your benzos for the comedown (again, clonazepam is preferable for this), that's when you
really need it.
In the future, if you take stimulants, I'd stay away from MPH, it's very dirty and side effect ridden, and the crash is a BITCH once you've abused it a few times. The only Rx stimulants commonly available that are "clean" are dextroamphetamine and it's somewhat non-abusable derivative, vyvanse, which has no comedown from what I've experienced, although the high isn't very impressive, it's main use to me is an all-day pick-me-up.
stimulation isn't a side effect, it's like the main effect of the drug, it's just a stimulant, not a euphoric stimulant by any means, unless you wanna to call the mental "high" that it gives you "euphoria", which I sure as hell wouldn't.