I've used phenibut many times, mostly on its own but also in conjuction with alcohol and benzodiazepines.
I have no expertise in pharmacology but I would say that the reason it potentiates alocohol is because of its GABA-ergic effects. Phenibut can be extremely sedating especially in high doses, useful for sleep but sometimes unpleasant. That said however, I once took ~14gm's of it in one go and as far as I know suffered no harm (apart from sleeping like a log for about 10 hours). So I would say that it is quite safe in overdose and it doesn't seem to repress breathing or anything like that.
Also, I doubt (going from personal experience) that there is any real risk with combining phenibut and alcohol (in moderation) compared to combining opiates or benzos with alcohol.
Phenibut is mainly active on GABA-B receptors while alcohol acts at GABA-A and in my experience the combination of these subtances is more synergistic rather than one potentiating the other.
A normal dose would be about 2g - 3g. However, some of the phenibut from certain manufacturers is a racemic mixture of the two enantiomers (one potent enantiomer and one not so potent) while other manufacturers use only the more potent enantiomer. If the product is labeled as phenibut XT then it's the more potent one, if its just labeled phenibut (no XT) then its probably a mixture. Obviously the dosage for the more potent products is lower.
Phenibut also affects the serotonin system in some way because the initial effects it produces (ie. half an hour or so after taking a dose) is one of slight "rushes" in your stomach and a kind of "loved up" feeling that I am familiar with from other serotonergic compounds (MDMA, BZP, Kanna, Psilocin).
Very little research has been done on the pharmacology involved with these effects so if you choose to use phenibut (therapeutically or recreationally) you should be aware of the possible risk of long term unobservable damage (ie. neurotoxic effects similar to other serotonergics, such as MDMA).
Another effect it has is repressing the release of PEA http://en.wikipedia.org/wiki/Phenylethylamine
, however, the day after taking a dose of phenibut you will notice what they call the "afterglow", this is where you will feel somewhat "tweaked" or stimulated, like the effects of a small amount of an amphetamine except it lasts the whole of the following day and can bring some slight anxiety with it. I woud attribute this "afterglow" to the release of all the PEA that had been accumulating while the phenibut was active in the body (but thats just speculation, like I said no-one knows exactly what is involved).
Phenibut also can cause tinnitus (ringing in the ears) but I've noticed this with alcohol as well and I would'nt worry to much about it.
Phenibut is somewhat like alcohol in that it's effects are often different from one dose to the next. For example sometimes it's very sedating other times stimulating without sedation.
So, like I said, it wont necessarily enable you to get "drunk" after two drinks (ie. pontentiating alcohol) but it will add to the buzz of the alcohol so that you won't feel the need to drink so much (ie. synergising with the alcohol) and you will sleep like a baby afterwards
As for other things to take to potentiate alcohol, I highly recommend cocoa powder (ie. chocolate powder). If you get some raw cacao (cocoa) powder (not the kind you buy in the supermarket. The kind you buy in a healthfood store) and add about one heaped tablespoon to a cup of hot water, then drink that about 30-40 minutes before you start drinking, it will seriously potentiate the alcohol, one pint of beer will have you reeling (in a good way), two pints will having you buzzin your *(#$ off!
I don't know what the mechanism with cocoa and alcohol is but cocoa does contain a lot of psychoactives which could be involved eg. theobromine, tyrosine, tyramine, PEA, anandamide, theophylline etc. It also has flavanols (types of antioxidants) in it which have been shown to cause vasodilation (expanding of the blood vessels) thereby increasing blood flow to the brain. This could be the reason it causes you to get drunk easier becuase there's more alcohol getting to the brain.
Again, if you decide to take cocoa with alcohol caution should be excercised eg. don't drink any red bulls because you will be over stimulated and could start hyperventilating and stuff.