I am doing a conversion of an old bianchi brava to FG. The rear spacing measures 129mm (im guessing 130 is the supposed measurement) and the hubs I have are 125 "adjustable". so my question here lies :

do i need to get spacers for these few millimeters or can I use bolt tension to pull the dropouts together. I was able to get the bolts to pull the dropouts to the right space without a wrench but will this cause any longterm problems? (I took the hub off an the spacing returned to normal.)

i believe i have the same problem as you. Not really a problem. I haven't used any spacers, just put the hub and tighten the bolts. Every time i loosen the bolts the dropouts go back to their normal position. Thats that i would say, unless someone has had a different experience.

another question I have is the rims say 700c/18c/23c do I get a tire thats 700/18 or 700/23?

You can just use bolt tension but the dropouts will probably be out of parallell from being pulled in and so your axle nuts may have a hard time gripping. Also the two sides may not come in equally, leading to tracking problems (note that I don't know this for sure, it's just a guess). It's really easy to add spacers to your axle; if you know how it will take five minutes and if you don't know how it will be a good lesson on how your hub is put together. I'd just do that.

Your rim might be stating minimum and maximum recommended sizes. 700*18 and 700*23 go on the same diameter rim, the 23s are just fatter. 23 is still just about as skinny as you'd want to go for a bike ridden off the track---it's what most road racers use. In practice you can exceed recommended sizes by quite a lot, as many cyclocross riders do, as long as you take extra care when mounting the tiire.

where can i get the spacers?

Your lbs will have em. They're a very inexpensive part, it should be well under a buck for them. You want two 2.5mm spacers. You can just use stainless washers that are the right size (id/od/ thickness), but the real thing is a bit simpler for a noob to wrap their head around. You will also need the correct size cone wrenches if you want to install them yourself. If you just get your lbs mech to do it it shouldn't cost much, my local guy would do it for $2 or so in the beer money jar.

There is NO reason for anyone to run 700x18 tires, even if you could find them.

I don't know if there's any reason not to do this, but if you're really lazy you could just throw the spacers on over the outer lock nut. It will be annoying every time you have to put your wheel between the dropouts, and you risk losing them when you remove your wheel. It has worked fine for me in the past, I have been known to do this for various reasons but usually only temporarily.

You can just use the bolts to pull the dropouts together, but I don't think it's the best idea. I have also done this with no ill effects but it's so easy to throw a couple of washers on that it's not even worth doing. Also, I think it would be a particularly bad idea with an aluminum frame.