Those are both nice bamboo, but they are at the edge of hardiness for your Zone, so you could get winter damage. If this concerns you, you might want to get something hardier, like Ph. nuda, or Fargesia nitida (new seedlings, because the older plants are in flower), or Fargesia murieliae.

You can never tell with culms vs. pot size: could be 1-3 culms for those sizes, and, if you're buying by mail, you're going to get plants that are topped, due to shipping size restrictions, and topped culms never grow taller. Sometimes, if the culms are young and bendy, you might get a 6' tall plant in a 4' high box.

After it's established and puts up new shoots, the culms will reach their mature height within about 2 months; new shoots each year will be fatter and taller until the plant maxes out. However, culms are not like trees...they reach their full size in the growing season and never get taller or fatter. If you have a 5' tall plant, you might get a 7' culm, and then an 8-10' culm the next year, etc. But remember, it will take a while to establish itself and you may not get any large new culms for the first couple of years.

A pond liner will not work as a barrier. I've seen rhizomes go right through the bottom of pond liners and cause leaks. I usually don't use barrier, but rhizome prune each year with a shovel. This may not work well for you in a paved area, so, if you're thinking of using barrier, get real bamboo barrier. Even then, it is not totally foolproof (I've seen an occasional penetration).

Bamboo Garden is well known, but there are many others, and there are American Bamboo Society (ABS) nurseries in NJ. If you can get to any of those, you can get bigger plants, get advice on hardiness for your area, and not have to deal with shipping charges. I've included a link for you of ABS plant suppliers. Scroll way down to see the listing of names by state, and then go to the alphabetical list to get details. If you still want to order by mail, send me an email and I'll give you a couple of other, reliable sources. Good luck.

Here is a link that might be useful: ABS Species Source list

32"x48"??????? Seems like a lack of space for a running bamboo. I would be suprised if they could ever muster 26' height in such small quarters. I wouldn't call any of the aureosulcata runners specifically erect. Here is quote from lewisbambo, inc website about spectabilis, "it grows quickly to 12 and 16 feet in height while remaining very bushy like with plently of low growing limbs"
Snowload can bend them to the ground and break some of their culms. Semiarundinaria fastousa is a more erect plant.

Bamboogarden is a good mail-order source, and like kudzu said, there are many others.

Also, if their will be bored dogs contained in this enclosure, most likely they will chew down the bamboo with no regard.